Russia warned on Monday that it was prepared to act unilaterally in Syria against groups that it said were breaking the cease-fire there, injecting a volatile new element into a conflict that has been calmer in recent weeks.

Russia’s Defense Ministry said the country’s military was ready to strike as early as Tuesday against groups that it said were violating the cease-fire unless U.S. leaders agree to discuss a Russian proposal for how to maintain the peace. So far, Russian warplanes have been observing the cease-fire, U.S. officials say.

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The ultimatum may be as much a negotiating gambit with the United States as it is a warning that Russia is about to act on the ground in Syria, from which it pulled a portion of its warplanes last week. The Russian military has sought close cooperation with the Pentagon in Syria; the Pentagon, angered by Russia’s actions both in Ukraine and in Syria, has held back. Secretary of State John F. Kerry is due to meet Thursday with Russian leaders in Moscow to discuss efforts to achieve peace in Syria.

Russian President Vladimir Putin announced last week that he would pull the bulk of Russia’s warplanes from Syria after their six-month deployment helped prop up Syrian President Bashar al-Assad. But as with other public announcements about Russian military deployments in recent years, the reality proved more complicated.

Russia’s Defense Ministry later announced it was flying as many as 25 sorties a day to support the Syrian army’s efforts to retake Palmyra from the Islamic State. That is permitted under the terms of the cease-fire deal. Such a tempo would require a significant deployment to remain in place in Syria.

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“We do not rule out the possibility that we will have to unilaterally use force to curb the actions of the rebels who fail to comply with the cease-fire arrangements,” the Russian Foreign Ministry said in a statement. The Russian Defense Ministry said earlier Monday that the action could come as quickly as Tuesday, should U.S. officials not respond to their proposals about how to address cease-fire violations.

The Russian Foreign Ministry said it was specifically concerned about groups that it says are allied with al-Qaeda’s affiliate in Syria, Jabhat al-Nusra. U.S. officials have complained that Russia and the Syrian government label all Assad opponents as Islamic State or al-Qaeda, even if they are more moderate.

The warning from Russia was a reminder of how tenuous the cease-fire remains, amid growing doubts that it can hold much longer. Observers say Assad forces have repeatedly violated the truce, including with barrel bombs Monday in several places around the country.

Opposition groups, meanwhile, say they are unwilling to hold to the cease-fire if it simply freezes the Syrian government’s recent territorial gains without offering any chance of a longer-lasting peace deal and political transition.

“We agreed to have a special mechanism” to handle cease-fire violations, said Russian Foreign Ministry spokeswoman Maria Zakharova. “In general, it was established, but in reality it doesn’t work.” She said the U.S. side had been slow to respond to Russian allegations of violations.

A Pentagon spokesman said there was no such delay.

“Whoever is making such statements must be misinformed, because these issues have been discussed at length already, and continue to be discussed, in a constructive manner,” Col. Steve Warren, a spokesman for the U.S.-led coalition in Iraq and Syria, said in an email. “We fully expect Russia to refrain from unilateral actions and respond to our ­counter-proposal” about how to put the truce agreement into effect, he said.

Missy Ryan in Washington and Hugh Naylor in Beirut contributed to this report.

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