WASHINGTON —The United States and Russia agreed Tuesday to try to convene an international conference on ending Syria's brutal civil war — possibly by the end of May — but the effort appeared to run into trouble within hours of its announcement with the key U.S.-backed opposition group reiterating that it won't attend talks involving top Assad regime officials.

The bid to revive a long-stalled peace plan, unveiled in Moscow by Secretary of State John Kerry and his Russian counterpart, Sergey Lavrov, reflected both sides' fears that worsening bloodshed, living conditions and waves of refugees are driving Syria to disintegration and threatening to plunge the region into sectarian mayhem.

"The alternative (to peace talks) is that there is even more violence," Kerry told reporters after his five-hour meeting with Lavrov. "The alternative is that Syria heads closer to the abyss, if not over the abyss and into chaos.

"The alternative is that the humanitarian crisis will grow," he added. "The alternative is that there may be even a breakup of Syria."

The pair met as the U.N. humanitarian agency said that the number of internally displaced people inside Syria had more than doubled to 4.25 million over the past two months, and two days after Israel stepped up its involvement with airstrikes on military targets in Damascus. Tensions also have soared over unproven allegations that chemical weapons have been used.

Kerry and Lavrov said they would push President Bashar Assad's regime and opposition leaders to attend an international peace conference that would seek to revive a June 2012 plan, known as the Geneva Communique.