U.S. Secretary of State John Kerry on Sunday held talks with Saudi Arabia's King Salman about the conflicts in Syria, Libya and Yemen, ahead of international meetings this coming week in Europe on those crises.

The visit by Kerry, who also held talks with the crown prince, deputy crown prince and foreign minister, comes at a critical time in efforts to rein in fighting and encourage political dialogue in all three countries, wracked by violence for years.

Kerry is trying to shore up the shaky truce in Syria that has been fraught with violations on both sides. While the U.S. and its partners accuse President Bashar Assad's government of the vast majority of breaches, they have acknowledged violations by the opposition.

The situation has been further complicated by the intermingling of some Western and Arab-backed rebels with groups such as the al-Qaida affiliate, known as the Nusra Front, which the U.N. has designated a terrorist organization and therefore not covered by the truce. Saudi Arabia and the U.S. have rejected attempts by Russia to get those rebels placed on the U.N. terrorist list.

Kerry was traveling later Sunday to Vienna where he planned to host, with Italy's foreign minister, talks on Libya on Monday, and, with his Russian counterpart, meetings on Syria on Tuesday.

The U.S. on Friday imposed sanctions on the speaker and president of Libya's House of Representatives for what the Obama administration said was their efforts to obstruct and delay political transition in the country. Since the 2011 uprising that toppled leader Moammar Gadhafi, Libya has been split between rival governments. Last year, the U.N. brokered a deal on a unity government to heal the rift among the Libyans. But the new government has so far failed to gain support from various factions.

The 17-member International Syria Support Group includes Turkey, Iran, Saudi Arabia and others. The August deadline that the U.S. has set for starting a political transition is a target, not a drop-dead endpoint for negotiations, Kerry has said.

The U.S. ultimatum has spurred speculation that if the deadline is blown, U.S. allies such as Saudi Arabia Arabia might respond by giving the Syrian opposition stronger weapons to fight Assad.

Kerry also has said indirect peace talks between the Syrian opposition and the Assad government probably would resume "some days" after the Vienna meeting. Those U.N.-led talks have been stalled since the opposition suspended participation last month in protest.

The U.N.'s humanitarian aid coordinator for Syria has expressed dismay about "disappointing" levels of access so far this month to besieged and hard-to-reach areas.

In Yemen, the warring parties have been holding U.N.-brokered negotiations in Kuwait to resolve the conflict. A truce began April 10 and has mostly held despite multiple breaches by both sides. The conflict pits the country's Shiite rebels, known as Houthis, and their allies against President Abed Rabbo Mansour's government, which is backed by a Saudi-led coalition.

The U.S. State Department said Kerry and Saudi officials discussed the need to strengthen that truce and their support for continued U.N. talks. A suicide bomber on Sunday detonated his explosives among policemen standing in line outside a police base in a southern Yemeni city, killing 25 people, security and health officials said. The Yemeni affiliate of the extremist Islamist State group claimed responsibility.

Kerry's trip also includes a visit to Brussels for a meeting of NATO foreign ministers before he flies to Asia to meet up with President Barack Obama in Vietnam.