Syrian regime warplanes unleashed relentless pre-dawn air raids Tuesday on rebel positions around Maaret al-Numan, a strategic northern town captured by insurgents last week, a watchdog said.

The raids were the "most violent" since the rebels took full control of the town in the province of Idlib last Wednesday, Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights, told AFP.

The United States on Monday called on all Syria's neighbors to keep a careful watch over their airspace, after Turkey said it had intercepted a Syrian plane from Russia carrying military equipment.

"Certainly we support the decision that Turkey has made in light of the apparent violation of their airspace by this aircraft," State Department spokeswoman Victoria Nuland told reporters.

German Foreign Minister Guido Westerwelle said Tuesday that Germany is "ready in principle" to host Syrians who have fled the civil war there, but added that it must be done under an international framework.

"Germany is ready in principle to welcome Syrian refugees," he told regional newspaper Rheinische Post.

Turkish Prime Minister Recep Tayyip Erdogan reiterated Monday that the cargo Ankara confiscated from an intercepted Syrian plane contained weapons, shrugging off Russian claims that the plane carried legal radar equipment.

"It is beyond any doubt that the cargo is war equipment," Erdogan told reporters in Ankara.

A Ukrainian journalist who worked for a Russian television crew and reportedly expressed support for Syrian President Bashar Assad's regime has been kidnapped by rebels, the Ukrainian authorities said on Monday.

"A journalist, a Ukrainian national who worked for Russian television channels, has gone missing in Syria," President Viktor Yanukovych said in a statement.

The Syrian army on Monday denied using cluster munitions and said it did not possess the weapon in its arsenal, in a statement published by state news agency SANA.

"Some news outlets that are complicit in the bloodletting in Syria have been publishing false reports that the Syrian army has been using cluster bombs against armed terrorists," it said, adding the military "does not have this kind of weapon."

A senior official on Monday said Iran proposed to peace envoy Lakhdar Brahimi a "transitional period" for Syria ahead of elections to be held under President Bashar Assad's supervision.

"We proposed a halt to the violence and a truce, stopping supplying weapons and backing for armed terrorist groups and a national dialogue between the opposition and government," Deputy Foreign Minister Hossein Amir Abdolahian told Arabic-language al-Alam television.

Clashes between Syrian troops and rebel fighters inside and outside the Umayyad Mosque in second city Aleppo have damaged the 13th century landmark, an AFP correspondent reported on Monday.

Fire has destroyed some of the antique carpets and wooden furnishings that used to adorn the place of worship and charred one of its intricately sculpted colonnades.

The majority of weapons secretly shipped to Syria at the behest of Saudi Arabia and Qatar go to hardline Islamic rebel groups rather than more secular organizations favored by the West, The New York Times reported Monday.

Citing unnamed officials, the newspaper said this was the conclusion reached in classified reports presented to President Barack Obama and other senior officials.

An Armenian plane ordered to land in Turkey Monday for security checks while en route to Syria's battered second city of Aleppo was allowed to resume its flight Monday after nothing suspicious was found, Anatolia news agency said.

"We know a plane from Armenia was forced to land in (eastern) Erzurum city... but it was allowed to resume its journey," the state news agency quoted Deputy Prime Minister Bulent Arinc as saying.