U.S. Secretary of State John Kerry speaks during a news conference in Amman on May 22, 2013. (Mohammad Hannon/AP)

The United States and its partners will widen support for Syrian rebels, potentially by sending more weapons or taking other measures short of sending American forces, if diplomacy fails to end a civil war that has killed “upwards of 100,000” people, Secretary of State John F. Kerry said Wednesday.

With the Syrian rebels he is trying to help in disarray, Kerry lowered expectations for an international effort to draw the opposition and a resurgent Syrian regime to the bargaining table. Top diplomats from the United States and 10 other nations supporting the rebels met here late Wednesday with Syrian rebel military chief Gen. Salim Idris and senior civilian opposition figures in preparation for peace talks.

They received a sobering summary of regime military gains and an influx of foreign fighters that threatens to turn Syria’s civil war into a regional conflict. The Syrian Observatory for Human Rights said this month that the death toll from the war is probably as high as 120,000, a significant jump from the United Nations’ February estimate of 70,000.

“These are big stakes, and I don’t have any illusions about how difficult it is to find that path forward,” Kerry said, speaking in Amman ahead of the strategy session with key Western and Arab nations backing the rebels.

The goal is to arrange an international peace conference next month in Geneva, under U.N. auspices, to negotiate a cease-fire and begin forming a transitional government.

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“In the event that we can’t find that way forward, in the event that the Assad regime is unwilling to negotiate in good faith, we will also talk about our continued support and growing support for the opposition in order to permit them to continue to fight for the freedom of their country,” Kerry said.

In a statement at the close of the session, the 11 nations represented said: “The Ministers expressed their strong concern over the increasing presence and growing radicalism on both sides of the conflict and terrorist elements in Syria, a matter that deepens the concerns for the future of Syria, threatens the security of neighboring countries and risks destabilizing the wider region and the world. The Ministers denounced the intervention of foreign combatants fighting on behalf of the regime, and consider their presence a flagrant intervention on Syrian territory and a serious threat to regional stability.”

Idris told the group that thousands of fighters from Iran, Iraq and the Lebanese militant group Hezbollah are now fighting alongside Syrian government forces, a senior State Department official said afterward. The official spoke on the condition of anonymity to discuss the closed-door session.

The United States and Russia, which back opposing sides in the conflict, have agreed to sponsor the larger peacemaking meeting and lobby the rebels and President Bashar al-Assad’s government to participate. The rebels have refused to commit to attending, while the government has been more receptive.

“We’re not here to dictate to them, or to anybody, an outcome,” Kerry said of the fractured rebel leadership. He promised that the 11 nations meeting in Jordan would “listen to all voices” about the format, agenda and timing of the proposed peace conference but warned that the process, if it takes hold, will not be swift.

The alternative to seeking a peace agreement is continued killing, the empowerment of extremists and the possible disintegration of Syria, Kerry said. So no matter the odds, he said, it is important to try.

“We believe that trying to get the Geneva process, difficult as it is, fraught with all the complications that it presents, is a better alternative,” he said.

Wednesday’s lead-up meeting hosted by Jordan included representatives from the United States, Jordan, Turkey, Saudi Arabia, the United Arab Emirates, Qatar, Egypt, Britain, France, Germany and Italy. The group represents the most active nations among a larger diplomatic group, known as the Friends of Syria, that has been unable to end the civil war.

Just before Kerry’s arrival, Syria’s ambassador to Jordan sought to upstage the U.S. diplomat with a news conference mocking the Friends of Syria countries and their efforts to reach a peace deal. Rather than Syria’s “friends,” they are the opposite, Bahjat Suleiman said. “The aim is to kill the people of Syria and try to fight its army and destroy its capabilities,” he said. “Those ‘friends’ did not stop at this point, but they have raced to arm, finance and sponsor these terrorist gangs and criminal clans.”Still, Suleiman suggested that peaceful negotiations are possible. “We are always willing to cooperate and collaborate with any opposition that has a program that serves the interest of Syria,” he said.

Jordanian Foreign Minister Nasser Judeh retorted that he hopes Suleiman “has enough diplomatic savoir faire not to criticize Jordan while accredited to Jordan.”

Judeh noted reports that Syria has begun circulating names of government officials who could participate in the peace talks. “Our firm position is that a political solution is the best solution that will lead to solving this crisis,” Judeh said after Kerry’s meetings with him and with Jordan’s King Abdullah II.

Both the Syrian government and the rebels appear to be intensifying fighting in a bid to gain ground ahead of any talks. Kerry acknowledged that Assad’s army had made advances recently but called them temporary.

“Yeah, he’s made a few gains in the last days. But this has gone up and down in a seesaw” and will continue to do so, Kerry told reporters. “And the fact is that support for the opposition is growing.”

The main opposition alliance on Wednesday urged fighters across the country to reinforce Qusair, a rebel-held town under attack by Assad’s troops and their allies from Hezbollah, the Associated Press reported. The town, near the Lebanese border, is a linchpin of a Syrian government military push to control a strategic backbone of territory connecting the capital, Damascus, to the Mediterranean coast.

Separately Wednesday, U.N. officials reported that an upswing in violence across southern Syria has prevented thousands of refugees from crossing into Jordan and warned of an impending humanitarian crisis along the ­Jordanian-Syrian border.

According to the U.N. High Commissioner for Refugees, intensified clashes and shelling along the 230-mile border have prevented thousands of Syrians from reaching Jordan since Saturday, shrinking into the single digits a refugee influx that at one point averaged about 2,500 people per day.

“From what we are seeing right now, the violence has cut off all access routes to Jordan,” said Andrew Harper, the UNHCR representative inside Jordan. “If thousands are continued to be displaced within Syria, we are very concerned that there is a large population stranded in southern Syria.”

According to Free Syrian Army officials, Assad’s forces have retaken several border towns, such as Tal Shihab, Khirbet Ghazaleh and Sahem al-Golan — key rebel strongholds that served as hubs for displaced people and key points on smuggling routes into Jordan.

Joby Warrick and Taylor Luck contributed to this report.