Syrian regime forces killed at least two civilians on Tuesday, as they kept up their bombardments of protest centers despite a warning by Washington that it is already discussing options in case a U.N.-backed peace plan collapses.

An advance team of U.N. military observers arrived in Damascus on Sunday to oversee a shaky ceasefire, but dozens of people -- troops as well as civilians -- have been killed since it went into force on Thursday, monitors said.

Iran will respond with force to any threats to its territorial integrity, President Mahmoud Ahmadinejad said on Tuesday, adding that it would prefer to cooperate with its Arab neighbors to maintain security in the Gulf.

"The armed forces and the army will inflict heavy regret and shame in case of any aggression against Iranian lands and interests," Ahmadinejad told military commanders and personnel on the occasion of Iran's annual Army Day.

Iran's foreign ministry on Monday summoned the Saudi ambassador to protest over Riyadh's treatment of Iranian nationals jailed in the kingdom, the official IRNA news agency reported.

Ministry officials questioned the ambassador over "worrisome" reports on the treatment of Iranian nationals jailed in the city of Dammam, capital of the Eastern Province that is home to most of the kingdom's Shiite minority.

A U.N.-Arab League peace plan for Syria has only a three percent chance of working, the emir of Qatar said on a visit to Rome Monday, as a U.N.-backed ceasefire was marred by ongoing violence.

Sheikh Hamad bin Khalifa al-Thani told a press conference that the chances for success "are no higher than three percent", and that the Syrian people should not be supported through peaceful means but "with arms".

A German shipping firm on Monday dismissed reports one of its vessels was carrying Iranian arms bound for Syria in violation of EU sanctions against the regime of President Bashar al-Assad.

Respected news weekly Der Spiegel reported at the weekend that the ship, the Atlantic Cruiser, owned by German firm Bockstiegel but chartered by a Ukrainian firm, had been stopped on the high seas with Iranian weapons on board.

The United States warned Monday that heightened violence in Syria threatens the sending of a full U.N. ceasefire observer mission as it voiced fear that a ceasefire is "eroding."

The U.S. ambassador to the United Nations, Susan Rice, said new attacks by government forces "call into question the wisdom and viability" of sending the full 200 international monitors.

Israeli authorities were on Monday holding 47 foreigners pending deportation, out of 79 barred from entering the Jewish state because of their links to a pro-Palestinian campaign, an official said.

"There are 47 people still awaiting deportation, including 37 French nationals, eight Britons, an Italian and a Canadian," immigration official Sabine Haddad told Agence France Presse.

The U.N.-appointed panel tasked with investigating rights abuses in Syria on Monday voiced concern over reports of post-ceasefire shelling by government forces.

The Commission of Inquiry on Syria said it had received accounts that the Khaldieh neighborhood and other districts of Homs had been bombarded since Thursday.

The United Arab Emirates warned on Monday that a Gulf islands dispute with Iran threatens "international security."

The UAE has summoned Iran's ambassador to Abu Dhabi to denounce a visit last week by President Mahmoud Ahmadinejad to one of the three disputed islands, the official state news agency said on Monday.

Syrian tribes opposed to the embattled regime of Bashar al-Assad said Monday they would join forces in an "Assembly of Tribes," Syrian opposition members and media reported.

"Today we proclaim, as children of all tribes, an Assembly of Tribes," opposition Syrian National Council (SNC) member Mahmut al-Maslat said in a statement read out on behalf of the assembly.