British forces are drawing up contingency plans for a “proportionate” response to an alleged chemical weapons attack in Syria, the prime minister’s office said Tuesday, while France said it is ready to “punish” those responsible, raising the possibility that the European nations could join a possible U.S.-led military strike.

Prime Minister David Cameron cut short a vacation to return to London and announced that he would recall Parliament to the capital on Thursday to discuss a possible military response.

In a televised address, French President Francois Hollande said it was the world’s responsibility to take action.

U.S. Defense Secretary Chuck Hagel told the BBC that U.S. forces are “ready to go,” but he reiterated that the United States wants to work “in concert” with the international community. “We have moved assets in place to be able to fulfill and comply with whatever option the president wishes to take,” Hagel said.

In Washington, White House spokesman Jay Carney told reporters Tuesday afternoon that President Obama has been consulting other world leaders but has not yet decided on a course of action in Syria. Carney said Obama has spoken with Cameron, Hollande, Canadian Prime Minister Stephen Harper and Australian Prime Minister Kevin Rudd.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

Obama “will continue to make calls to his counterparts throughout the week,” Carney said. “Nothing has been decided,” he added.

In Syria, meanwhile, Foreign Minister Walid al-Moualem said Syria’s military would defend the country against any foreign intervention. Syria vehemently denies responsibility for the attack, which left more than 300 people dead and many wounded.

“We all hear the drums of war around us,” Moualem said. “If they want to attack Syria, I think that using the lie of chemical weapons is fake and not accurate, and I challenge them to show evidence.”

He said the idea of a Western military strike to change the balance of power in Syria, which has been embroiled in a vicious civil conflict for more than two years, is “delusional and not at all possible.”

Moualem said Secretary of State John F. Kerry called him Thursday, their first contact in 2 1/2 years. During a “friendly” conversation, Moualem said, Kerry requested that a U.N. team in Syria be allowed access to areas where the strikes allegedly happened.

The U.N. inspectors gained access Monday to one of the sites of last week’s alleged chemical attack and spent three hours interviewing witnesses and gathering evidence, despite its convoy briefly coming under sniper fire earlier in the day.

Following the incident, a second day of investigations was postponed Tuesday. Moualem said the team was not able to visit a second site because rebel groups could not guarantee security. Khalid Saleh, a spokesman for the Syrian Opposition Coalition, blamed the Syrian government, saying it had not allowed the U.N. team to leave its hotel, citing “security reasons.”

The United Nations declined to comment on why the team was delayed, saying that casting blame could jeopardize negotiations over safely reaching the areas struck by the attack.

“I’m not going to point fingers,” said U.N. spokesman Farhan Haq. “There is a complex situation on the ground, and we are trying our best to secure access.” He said he expected progress in the “next few hours.”

Moualem said the government of President Bashar al-Assad was cooperating fully with the probe and that the United Nations had requested access to the sites in question only on Saturday.

However, Haq said the initial request was sent to the Syrian government on Thursday, the day after the alleged attack.

In retaliation for the “massacre” in the Damascus suburbs, al-Qaeda-linked rebels said Tuesday they would strike Assad’s security branches and infrastructure in an operation called “Volcano of Revenge,” according to a statement distributed by the jihadist Baqiya Media Foundation.

“It has become proven to us that this enemy only knows the language of force, and we will discuss with it with rocket launchers and shells and make them to be lava over its head,” said the statement, which was signed by the Islamic State of Iraq and the Levant, the recently expanded al-Qaeda in Iraq, the hard-line Islamist Ahrar al-Sham rebel group and seven other rebel factions.

Jabhat al-Nusra, classified as a terrorist organization by the United States, has also vowed “vengeance” through attacks on villages inhabited by Alawites, a Syrian religious minority to which Assad belongs.

The British, along with the French, have led the calls in Europe to boost efforts to aid the Syrian opposition in its fight against Assad. France said Tuesday that its forces were ready.

“France is ready to punish those who took the decision to gas the innocent,” Hollande said.

Cameron, however, is facing mounting pressure — including from within his own Conservative Party — to win support in Parliament before taking any action.

Cameron has the authority to launch strikes with or without the backing of Parliament. But any attempt to intervene in Syria over the objections of lawmakers would be politically risky. Thus, he is likely at least to make a vigorous attempt to win over lawmakers before opting to have Britain join any military operation.

Some lawmakers remain skeptical about whether further involvement in Syria’s civil war suits British national interests, especially without a U.N. mandate.

“I’m going to need convincing,” said Andrew Bridgen, a Conservative member of Parliament who drafted a letter to Cameron signed by 81 lawmakers. He said he was puzzled as to why the Syrian regime would choose to use chemical weapons at this point in the civil war, knowing the likely stakes.

“We need to hear there are limits to how our involvement could escalate,” Bridgen said. “Our military is already stretched and battled-fatigued.”

Richard Kemp, former commander of British forces in Afghanistan and a London-based defense analyst, said in an interview that Britain would involve itself in limited military action in Syria only if it were a “multinational” effort. He also said that Syria’s sophisticated air defense system may need to be “neutralized” to allow missiles to hit their marks and that British forces on the Mediterranean island of Cyprus could “undoubtedly” play a role if London decided to participate in strikes.

The British base on Cyprus “could be used for launching aircraft or used for command and control, or it could be used for intelligence gathering,” Kemp said.

Russia, which is Assad’s strongest ally, has repeatedly blocked action against Syria at the U.N. Security Council, leaving few palatable options for Obama, who values the backing of international institutions.

But the powerful evidence that chemical weapons were used in rebel strongholds in the suburbs of Damascus last week — including multiple videos of children gasping, drooling and dying — has abruptly shifted the discussion on whether military intervention is an option.

Doctors Without Borders, the French relief agency, said three hospitals it supports in the afflicted areas had treated 3,600 patients who were displaying neurotoxic symptoms, of whom 355 died.

The Syrian government has blamed the rebels for the attack and says some of its troops also have been affected by an unspecified nerve gas.

However, the Syrian Opposition Coalition’s Saleh said that during a meeting with envoys from the 11 “Friends of Syria” countries in Istanbul on Monday, there was broad agreement that the Syrian government was responsible.

Saleh denied reports that Western powers told the opposition during the meeting to expect a strike “within days.” Instead, the spokesman said, the coalition was informed that a decision has not yet been made.

In a phone call to Russian President Vladimir Putin on Monday, Cameron made clear that he holds the Assad government responsible. “There was no evidence to suggest that the opposition had the capability to carry out such a significant attack, and the regime had launched a heavy offensive in the area in the days before the incident,” a Downing Street spokesman said.

U.S. Secretary of State John F. Kerry also pointed the finger squarely at Assad’s forces on Monday, saying that the use of chemical weapons was “undeniable” and that the Syrian government’s decision to allow inspections was “too late to be credible.”

The Obama administration has already said it has evidence that the Assad regime has used chemical weapons on several occasions, but the scale of this attack, if confirmed, would unequivocally cross a “red line” set by the president a year ago.

Kerry has described the attack as a “moral obscenity.”

Morris reported from Beirut. Karla Adam in London, Ahmed Ramadan in Beirut and William Branigin in Washington contributed to this report.