US President Barack Obama departs the White House for his trip to Saudi Arabia, Germany and Great Britain | Molly Riley/AFP via Getty Images Obama sees another foreign policy goal crumble in Syria Lawmakers offer an ‘I told you so’ as a fragile cease-fire veers toward collapse.

President Barack Obama's most tangible success in his push to stop Syria's bloodbath — a cease-fire struck in late February — is rapidly turning into a failure. His fiercest critics in Congress, meanwhile, are basically saying, "I told you so."

The always-fragile truce "may be breaking down" the president acknowledged Thursday while visiting Saudi Arabia. And he still sounded more optimistic than others.

Syrian opposition leaders, angry over Syrian President Bashar Assad's renewed airstrikes against rebel strongholds, are using terms like "buried" to describe the cease-fire. The Soufan Group, a respected intelligence consulting firm, declared Thursday that the cease-fire has "effectively collapsed."

In statements to POLITICO, U.S. lawmakers from both parties warned the truce was "unraveling" and "deteriorating." They noted that Syrian peace talks, too, have stalled in recent days as the Syrian opposition has essentially pulled out due to its anger over Assad's actions.

"We are at grave risk of a complete resumption of hostilities, if not an even worse escalation," said Rep. Adam Schiff (D-Calif.), the ranking member of the House Intelligence Committee. "All efforts must be made to reverse this trend so that the international community can work towards a political solution."

John McCain, chairman of the Senate Armed Services Committee and a longtime critic of Obama's largely hands-off approach to Syria, called the failing cease-fire "predictable," before quoting the late German leader Otto Von Bismarck.

"In the words of Mr. Bismarck, 'The issue will be decided by blood and iron,'" the Arizona Republican said.

Obama and his aides have taken steps in recent days to try to shore up the cease-fire, which, for legal and technical reasons they refer to as a "cessation of hostilities." The fact that the truce held at all was a major relief to an administration worried that its foreign policy legacy will be defined by the relentless war in Syria, which has killed at least 250,000 people.

Obama on Monday held what the White House described as an "intense conversation" with Russian President Vladimir Putin, a supporter of Assad's. Obama urged Putin to persuade Assad, who also has the backing of Iran, to stop his attacks on opposition strongholds.

In a statement afterward, Putin said he was committed to strengthening the cease-fire but he also stressed the "need for the moderate opposition to distance themselves swiftly" from terrorist groups like the Islamic State. Assad and Russia have frequently insisted that Syrian forces are targeting such "terrorists" when U.S.-supported rebels have been hit.

Russia's support of Assad has been a huge strain on an already bitter relationship between Obama and Putin. The Russian president, who also invaded Ukraine on Obama's watch, has long viewed Obama as naive about the harsh realities of the Middle East, while Obama sees Putin as destined to get bogged down in quagmires,

Just weeks ago, in a move that surprised the White House, Putin announced he was withdrawing his military troops from Syria, where they had been providing air cover to Assad's ground forces. But reports since indicate the withdrawal has been partial at best, angering U.S. lawmakers who never trusted Moscow in the first place.

"Russia will continue to violate the ceasefire as long as Putin knows he can get away with it. Clearly, the Obama administration has failed to convince Russia, Iran and the Assad regime that continued attacks on the Syrian people will have consequences," said GOP Rep. Ed Royce of California, who chairs the House Foreign Affairs Committee.

On Thursday, Deputy National Security Adviser Ben Rhodes said the administration is "concerned about reports of Russia moving [military] materiel into Syria."

"We believe ... that Russia should focus its efforts on the diplomatic process, on maintaining the cessation of hostilities and working with the Syrian government to get them to take seriously the process of negotiation and, ultimately, [political] transition," Rhodes said.

Even as he held firm on the notion that Assad must step down as Syria's leader, Obama gave no sign that he would consider sending ground troops to help resolve the conflict in the Arab state, other than the Special Forces currently fighting the Islamic State. With nine months left in his presidency, Obama is hesitant to commit more American lives to a conflict with no real end in sight, even if it means historians will criticize him.

The president said that, should the cease-fire collapse, there's no real back-up plan other than pressing forth with attempts to reach a political agreement.

"The problem with any Plan B that does not involve a political settlement is that it means more fighting, potentially for years," Obama said.

Jeremy Herb and Seung-Min Kim contributed to this report.