WASHINGTON (AP) — Secretary of State John Kerry is headed back to the Middle East to press his case for peace talks between Syrian rebels and President Bashar Assad's regime amid increasing signs the new U.S. strategy to halt the war is being undermined by Russia.

Kerry left Washington on Monday for Oman where he will have discussions with the sultan of the Gulf state. He will then travel on to Jordan to gather with 10 of America's closest European and Arab partners to discuss how to advance a political transition and end more than two years of bloodshed in Syria, before traveling on to Israel.

For the Syria negotiations to succeed, the Obama administration is banking on Russia's help.

The U.S. and Russia have wrangled repeatedly while more than 70,000 Syrians have died, but they now say they're working together to start direct talks between Syria's government and the opposition in Geneva next month. Washington demands Assad's ouster, while Russia continues to provide the Syrian leader with military aid and diplomatic cover, but President Barack Obama this week said the meeting "may yield results."

The optimism echoes the message of Kerry, who during his Moscow visit earlier this month declared that the old Cold War foes, by rejuvenating Syrian peace hopes, were demonstrating how they "can accomplish great things together when the world needs it."

For all the heady talk of cooperation, however, Russia has continued to rebuff American demands that it cut off military support for Assad.

Moscow is preparing to give Syria state-of-the-art ground-to-air missile systems, Israeli officials say. It is beefing up its naval presence near its base in northwestern Syria, reports suggest. And, in the latest revelation, U.S. officials say Russia has provided the Assad regime with anti-ship cruise missiles.

Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, said the transfer of the advanced anti-ship missiles is "an unfortunate decision that will embolden the regime and prolong the suffering."

On the diplomatic front, the situation isn't much better. There, Russia has repeatedly blocked a proposal for an expanded Security Council trip to Turkey and Lebanon to study Syria's refugee crisis, according to U.N. diplomats.

The continued friction between Moscow on the one hand and Washington and its partners on the other comes as the Obama administration is evaluating a range of options, including military ones, to break the stalemate in Syria's civil war and respond to evidence that Assad's forces used small amounts of chemical weapons in two attacks in March. Obama previously declared chemical weapons use his "red line" for a more forceful American intervention, though Kerry and other U.S. officials have since suggested that no such step would be taken while the new peace push still has hope.

Russia's missiles support significantly boosts Syria's capability to target manned planes, drones and incoming missiles after its systems were easily circumvented in 2007 when Israeli jets bombed a suspected nuclear reactor site along the Euphrates River in northeastern Syria. Apparently successful Israeli strikes in recent weeks on weapons convoys to Hezbollah show the Syrian defenses are still far from impregnable, but the new weaponry adds further considerations as the United States tries to change Assad's calculation that he can prevail in Syria's civil war.

While more and better anti-missile systems wouldn't immediately change the fight between Syria's government and armed opposition, they would make it more dangerous for the U.S. and other governments to try to enforce a no-fly zone in the country or otherwise intervening militarily. And with Washington mulling over the options, the war continues. The refugee toll has topped 1.5 million people and much of the country has slipped into lawlessness.

Kerry's weeklong trip will also see him try to advance his two-month effort to restart peace talks between Israel and the Palestinians.

The secretary has persuaded the Arab world to help by sweetening its deal of universal recognition for the Jewish state if it pulls out of most of the territory in east Jerusalem and the West Bank that it conquered in the 1967 Mideast war. But he has struggled to gain any public concession from Israel, which was accused of taking steps last week to legalize four unauthorized settlement outposts in the West Bank. The Palestinians see that land as part of its future state.

Kerry also will travel to Ethiopia to celebrate the 50th anniversary of the founding of the Organization of African Unity, the precursor to today's African Union. While in Addis Ababa, Kerry plans a separate one-on-one meeting with Egyptian President Mohammed Morsi, officials said.

In Oman, Kerry will talk with officials about their plans to purchase a $2.1 billion air defense system from American manufacturer Raytheon.