U.N. Secretary General Ban Ki-moon on Tuesday raised alarm over a $100 million shortfall in funding for the U.N. aid agency for Palestinian refugees and called for urgent donations.

Ban said in a statement that the UNRWA relief agency was "a pillar of stability" for five million Palestinian refugees at a time when the Middle East is in the throes of crises and suffering.

Syrian Foreign Minister Walid Muallem arrived in Iran Tuesday to meet his Iranian counterpart Mohammad Javad Zarif to discuss the situation in the region, the official IRNA news agency reported.

His visit comes with Russian Deputy Foreign Minister Mikhail Bogdanov, President Vladimir Putin's special Middle East representative, also in Tehran.

A Syrian military aircraft that crashed during a bombing run over rebel-held territory killed at least 37 people and injured more than 60, a monitor said on Tuesday.

The victims died when the plane crashed on Monday in the town of Ariha in Idlib province, and in preceding air strikes, according to the Syrian Observatory for Human Rights, which originally said 31 were killed.

A U.S. warning that it would use its air power to defend Pentagon-trained rebels against Syrian troops was met with skepticism by officials in Damascus on Tuesday.

The U.S.-backed Division 30 was also left reeling when the Al-Qaida affiliated Al-Nusra front captured at least five of its members in overnight raids, said the Syrian Observatory for Human Rights.

Life in the Syrian city of Hasakeh, divided between allied Kurdish and regime forces, comes at a price: two lots of military service and double the taxes.

Raed, a Syrian Arab living in the northeastern city, avoids passing through checkpoints run by the Kurdish People's Protection Units (YPG).

The United States on Monday said it has used air power in Syria in defense of allied rebel groups, signaling deeper involvement in the country's brutal four-year civil war.

The Pentagon confirmed that an air strike was carried out Friday in support of the New Syria Force, a U.S.-allied group. 

Russia's Foreign Minister Sergei Lavrov on Monday dismissed as "counter-productive" an announcement by Washington that it could take extra measures to defend U.S.-allied fighters in Syria.

At a news conference in Qatar, Lavrov condemned comments by the White House that it could take "additional steps" to protect allies in Syria and warned Bashar Assad's regime not to impede their actions.

The White House said Monday said it is willing to take "additional steps" to defend U.S.-allied fighters in Syria, warning the regime of Bashar Assad not to impede their actions.

Spokesman Josh Earnest said Syria "should not interfere" with operations by U.S.-trained forces, warning that "additional steps" could be taken to defend them, raising the prospect of strikes against the regime.

A Syrian army aircraft crashed during a bombing run over rebel-held Ariha on Monday, killing at least 31 people and destroying a vegetable market, a monitoring group and residents said.

The Syrian Observatory for Human Rights said it was not immediately clear how many of the deaths in the northwestern town had resulted from the crash and how many were killed in the bombing run.

Russian President Vladimir Putin is having a change of heart on the Kremlin's wholehearted support for Syrian leader Bashar Assad and may "give up on him" in the future, Turkish President Recep Tayyip Erdogan was quoted as saying Monday.

When asked if Putin could be persuaded not to support Assad, Erdogan said he saw his counterpart as "more positive" during a face-to-face meeting in the Azerbaijani capital of Baku in June and in subsequent telephone talks.