Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

Syria’s embattled political opposition said Friday that there is little point in peace negotiations while Syrian President Bashar al-Assad’s military is using help from Hezbollah and Iran to make rapid military gains against rebel forces.

“We need to change the balance of power on the ground” ahead of proposed talks, Khalid Saleh, spokesman for the umbrella Syrian Opposition Coalition, said in an interview in Washington. “We need our friends to start putting more pressure” on Assad’s forces by using airstrikes and supplying weapons to rebel forces, he said.

But while Saleh predicted that the United States, Britain and France were all on the verge of changing their nonintervention policies, none of those countries has yet made a commitment to provide arms or other lethal help to the rebels.

Peace talks between the Syrian government and the opposition were jointly proposed early last month by the United States and Russia, Assad’s primary arms supplier. But the conference has been put off until at least July amid intensified fighting inside Syria and disagreements about the meeting.

U.S. and Russian representatives met this week in Geneva but failed to set a date or reach an accord on who should attend. Both the United Nations — the official sponsor of the proposed talks — and Russia have said that Iran should be among about a dozen outside governments present, according to a senior Obama administration official who spoke on the condition of anonymity about the diplomatic wrangling. The United States, backed by opposition supporters in Europe and the Middle East, said Iran’s presence would not be helpful and proposed alternatives that Russia and the United Nations rejected.

While Assad’s government has agreed to attend, the opposition has not.

Acting coalition head George Sabra said that only rapid international intervention would stop what has become “a sectarian war between Sunnis and Shiites. The problem will [spread] all over the Middle East, to Lebanon, to Turkey, to Iraq, to Jordan and maybe to the [Persian] Gulf,” Sabra said in an interview with the Associated Press.

Sabra spoke after Assad’s forces, with help from Hezbollah, the militant group based in Lebanon and supported by Iran, this week captured the rebel-held town of Qusair. Hezbollah, Iran and Assad have charged that Sunni extremists within the opposition are seeking to control the country.

In the absence of a U.N. Security Council resolution condemning Assad’s forces for the suspected use of chemical weapons and heavy offensive weapons against civilians, the rebels have called for Western or Arab airstrikes on Syrian chemical and missile sites and a possible no-fly zone over rebel-held areas.

The Obama administration has said such action is possible, depending in part on whether the White House determines that Assad used chemical weapons. France recently sent its dossier of evidence of such use to the United States, but the State Department said this week that there is no final determination by Washington

Saleh, the coalition spokesman, predicted that the United States was only “weeks away” from taking such action, and he said that Britain and France may move even sooner. But that timetable appeared optimistic.

Although a European Union embargo on weapons shipments to Syria expired last week, British Prime Minister David Cameron said Friday that his country’s Parliament would be given a chance to debate and vote on any decision. Opposition in Britain is high even within Cameron’s own Conservative Party.

U.S. momentum toward providing lethal assistance to the rebels was slowed when Secretary of State John F. Kerry agreed with his Russian counterpart to move toward talks at a time when others are pushing for action.

Sen. John McCain (R-Ariz.), who has led the charge in Congress for intervention, argued Thursday that the United States has a duty to do more militarily to stop the disintegration of Syria and the rise of extremist groups.

“Assad has turned the tide of battle on the ground. His foreign allies have all doubled down on him,” McCain said in an address at the Brookings Institution. “Iran is all in. Russia is all in. Shia militants are flowing into the fight from Iraq, and Hezbollah fighters have invaded Syria by the thousands.”

As desperate civilians have tried to flee Qusair and other sites of intense fighting, the United Nations made the largest humanitarian appeal in its history Friday, saying that half the Syrian population — more than 10 million people — would need aid by the end of the year and increasing its projection of needed aid to at least $5.2 billion. The previous appeal for $3 billion has collected less than half that amount.

U.N. humanitarian chief Valerie Amos said the situation in Syria has “deteriorated drastically,” with violence intensifying in most areas of the country. According to U.N. estimates, more than 4 million people have been internally displaced during the two-year-old conflict, documented refugees in neighboring countries total 1.6 million, and schools and hospitals have been among the military targets.

Loveday Morris in Beirut contributed to this report.