WASHINGTON -- The Obama administration has another chance to enforce its botched "red line" against the use of chemical weapons in Syria, given new reports that President Bashar Assad's regime has used nerve gas against extremist fighters and may be planning more such attacks.

Obama's decision not to retaliate against Assad's use of chemical weapons in 2013 has become an emblem for his larger foreign policy, which critics argue hasn't been forceful enough in Syria and other places. Obama justified his restraint by citing the diplomatic agreement that was brokered by the U.S. and Russia to destroy Syria's chemical arsenal. But new Israeli reports question whether Assad has complied.

The Israeli newspaper Haaretz, apparently relying on a government source, reported on May 2 that Assad's forces used sarin gas in late April against Islamic State fighters after they attacked two Syrian air force bases east of Damascus. Stockpiles of this deadly gas were supposed to have been removed from Syria in 2014.

Given the international silence, Israeli officials are said to fear that Assad will keep striking with the banned weapons. "With the continuation of fighting in Syria, it is reasonable to assume that the regime won't hesitate to use these weapons again, especially after already having done so ... without any reaction," an Israeli source told me.

The alleged use of sarin is another sign that Assad appears ready to breach any diplomatic efforts to de-escalate the war. In recent weeks, his forces, backed by Russia, have struck a hospital in Aleppo run by Doctors Without Borders, a pediatric hospital there, and a U.S.-backed humanitarian group in Idlib called Syria Civil Defense.

Chemical weapons have become part of "the new normal" in Syria, according to a report in February by the Syrian American Medical Society. The group said that in 2015, there were 69 chemical weapons attacks in Syria, mostly chlorine bombs dropped by Assad's air force.

The Assad regime often justifies such attacks by saying it is bombing the Islamic State or Jabhat al-Nusra, the al-Qaeda affiliate in Syria. But these jihadists are intermingled with civilians and moderate opposition groups in ways that make the non-extremist groups targets, too. As Assad has pressed his campaign in Aleppo and elsewhere, the "cessation of hostilities" negotiated by the U.S. and Russia in February has frayed badly.

The possibility that Syria retains chemical weapons was noted recently by Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons. "There are still questions. I am not able to say whether Syria has declared everything or whether Syria continues to possess some chemical weapons or some munitions," he cautioned. Uzumcu also noted "extremely worrying" signs that the Islamic State has used mustard gas in Syria and Iraq.

Obama administration officials are concerned about continued Syrian use of chemical weapons, but they see significant differences between the recent reported incidents and the size and scope of the 2013 attacks using sarin and VX, which are believed to have killed more than 1,400 Syrian civilians.

Diplomacy remains the administration's focus in Syria -- and the partnership with Russia seems to be expanding, rather than shrinking, despite its setbacks. To bolster the cease-fire, U.S. and Russian officials have been discussing the location of "protected" Syrian opposition groups. Officials from the two countries are said to talk daily in Geneva and by telephone to Syria, arguing over which areas are legitimate extremist targets and which should be avoided. This shared "domain awareness," as one official describes it, illustrates the extent of quiet Russian-American cooperation.

But Syria shows the limits of this great-power diplomacy. Russia can't seem to control Assad, even when it attempts to do so. And the U.S. has been unable to force opposition fighters to disentangle themselves from Jabhat al-Nusra and the Islamic State. Assad, once seen as a mild-mannered ophthalmologist, has proved a headstrong, brutal leader who has spawned the equivalently vicious Islamic State.

Finally, there remains a gaping hole in the U.S. strategy for capturing the Islamic State's strongholds in Raqqa and Manbij in eastern Syria. Washington wants this fight to be led by Sunni Arabs, but the only reliable fighters America has found are Syrian Kurds from the YPG militia -- which, to complicate matters further, is viewed by Turkey (a NATO ally) as a terrorist group.

Who will bell this cat? Are Presidents Obama and Putin really ready to tolerate a situation where the use of chemical weapons is seen as "normal," despite a Russian-American agreement that they should be banned?

(c) 2016, Washington Post Writers Group