GENEVA — The Latest on the war in Syria (all times local):

Russia's military says Syrian forces backed by Russia aircraft turned back an attempt by a large group of rebels advancing toward the city of Palmyra.

Five rebels were killed in the Monday morning clash near Al-Tanf, said a statement from the Russian center for reconciliation of the warring parties in Syria.

A garrison for the U.S.-led international military coalition is located in Al-Tanf, where fighters opposing the Islamic State group are trained.

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1:30 p.m.

The U.N. humanitarian agency's top official for Syria is expressing concern about recent airstrikes in rebel-held Idlib province, warning that "we may have not seen the worst of the crisis" in Syria even after 7 years of war.

Panos Moumtzis of U.N. OCHA says military escalation could make Idlib's situation "much more complicated and brutal" than other conflict zones in Syria.

He said Monday that 920,000 people were displaced within Syria between January and April, the highest number of any four-month span since the war began.

OCHA is on "high alert" for possible further displacement among the estimated 2-1/2 million people in Idlib, adding "There is no place left to go" within Syria.

The Syrian Observatory for Human Rights says airstrikes have killed dozens in towns in Idlib since Wednesday.