Oren Dorell

USA TODAY

Secretary of State John Kerry warned Tuesday of the prospect of a more violent Syria if a cease-fire agreed to with Russia fails to lead to a political transition for the war-torn country.

Facing skeptical lawmakers at the Senate Foreign Relations Committee, Kerry referred multiple times to a “Plan B” alternative to diplomacy and said “this could get a lot uglier” for Russia if the agreed-to process fails.

Kerry said he agrees with Gen. Philip Breedlove, commander of the U.S. European Command, who said last week that Russia is “weaponizing” migrants to put pressure on Europe and the West. Kerry said half of the more than 1 million migrants who flooded into Europe in 2015 are Syrian, and Europe cannot bear to take in another 1 million migrants if the Syria conflict does not end.

Kerry appeared before the committee he once chaired to seek approval to spend $50.1 billion on diplomacy and international assistance in 2017. It was his last budget request as the top U.S. diplomat before the end of President Obama’s presidency.

Republican and Democratic senators questioned the administration’s approach to Syria and a more threatening Russia. The hearing took place after Syria's embattled President Bashar Assad announced he would agree to the temporary "cessation of hostilities" that the United States and Russia announced Monday.

That temporary cease-fire is to begin Saturday, but it excludes the Islamic State and Nusra Front terrorist groups. The Pentagon said the agreement won't impact the U.S.-led air campaign targeting the Islamic State in Syria. A previous temporary cease-fire agreement made by the United States and Russia that was to begin last Friday failed to take place.

Syria state media: Assad accepts U.S.-Russia cease-fire deal

Russian airstrikes in Syria that started in September have changed the course of the five-year Syrian civil war in favor of Assad and his supporters. Russia says it attacks the Islamic State and other terrorists, but the United States says most of its targets are fighters for the legitimate opposition to Assad.

The Syrian conflict has caused about 470,000 deaths, according to a new study by the Syrian Center for Policy Research. The war has displaced more than half of Syria’s population of 22 million. The refugees have exacerbated a migrant crush impacting Syria’s neighbors and causing discord between members of the European Union.

EU ultimatum to Greece: Stem flow of migrants through to northern Europe

A mind-boggling stew of nations is fighting in Syria's civil war

“I don’t think I’ve seen Europe so unsettled ever in my lifetime,” said Sen. Bob Corker, R-Tenn., chairman of the committee. Russia is “trying to destabilize the area, using refugees as a weapon of war.”

Kerry agreed and said he told Russian President Vladimir Putin that time for diplomatic progress is limited.

"We’re going to know in a month or two whether or not this transition process is really serious,” Kerry said he told Putin when the two met. “Assad himself is going to have to make some decisions and to show that they’re serious. If there isn’t, there are certainly Plan B options that will have to be considered.”

Sen. Barbara Boxer, D-Calif., who praised the Obama administration’s policies in Cuba and Africa, questioned whether Russia is a trustworthy partner in Syria.

“We have the opportunity to put to the test their commitment to a political solution,” Kerry said. “If this doesn’t end and the opposition continues to fight, this could get a lot uglier. The Russians have to consider that, too.”

Kerry’s warnings came as Turkey and Saudi Arabia said they will take action against Russian and Syrian government attacks on Syrian opposition forces.

Saudi Foreign Minister Adel al-Jubeir told Germany’s Der Spiegel newspaper last week that Syria’s moderate opposition should be provided anti-aircraft missiles “to change the balance of power on the ground.” The impact would be similar to what happened in Afghanistan during the Soviet Union’s failed occupation in the 1980s, al-Jubeir said.

Such options are known to be “out there,” and Putin is smart enough to know the war will not end as long as Assad is in power, Kerry said.

Sen. Rand Paul, R-Ky., asked why Kerry supports the opposition's insistence that Assad should not participate in elections.

“Sure, he’s a terrible guy, but the Middle East is full of them,” Paul said. Under the Russian-Syrian onslaught, the opposition is “being overrun as we speak.”

“You cannot impose Assad on them after everything he’s done to them,” Kerry said.