At least five people were killed in Syria, activists said, despite an order from President Bashar Assad for security forces not to open fire on protesters and an offer of dialogue.

Friday's bloodshed cast a pall over the government's pledges to forge ahead with reforms in Syria, which has been gripped by two months of deadly protests.

Suspected al-Qaida rebels ambushed an army vehicle and killed five soldiers near the Yemeni town of Marib, east of the capital Sanaa, on Friday, a security official told Agence France Presse.

"The vehicle was ambushed with an RPG (rocket-propelled grenade) and all five soldiers inside died," the official said. "Al-Qaida is suspected of carrying out this attack."

Russian Foreign Minister Sergei Lavrov warned Friday against foreign intervention in Syria, calling on the Syrian opposition not to seek a repeat of the "Libya scenario".

"We are very worried that the process of reconciliation, the process of the start of dialogue ... is being slowed down by a desire of some participants to attract foreign forces to support their actions," Russian news agencies quoted Lavrov as saying.

Israeli security forces were on high alert on Friday for fear of violence as the Palestinians begin marking the "Nakba" or "catastrophe" which befell them following Israel's establishment in 1948.

"The police are on high alert and we have deployed thousands of police officers in and around Jerusalem, as well as in the north," police spokesman Micky Rosenfeld told Agence France Presse.

Gunmen killed three protesters tearing up posters of President Ali Abdullah Saleh on Thursday, raising to 19 the death toll in 24 hours and prompting Washington to say the time has come for a transfer of power.

Pro-Saleh gunmen on the roof of the ruling party's headquarters in al-Bayda, 210 kilometers southeast of Sanaa, opened fire at demonstrators tearing up posters of Saleh, witnesses and protest organizers said.

U.S. Secretary of State Hillary Clinton on Thursday blasted Syria's use of strength as "a sign of remarkable weakness" at a news conference in the Greenland capital.

Syria's repression of protests "is a sign of remarkable weakness," Clinton told reporters at an Arctic Council meeting, stressing Syria continued with "a brutal crackdown" on demonstrators despite overwhelming international condemnation.

British Prime Minister David Cameron invited on Thursday the Libyan rebel council to open an office in London after talks with its leader Mustafa Abdul Jalil.

It would be the National Transitional Council's first foreign diplomatic mission and a highly symbolic step in the rebels' fight against Libyan leader Moammer Gadhafi, who was shown on television Wednesday for the first time in nearly two weeks.

A blazing row has erupted between President Mahmoud Ahmadinejad and parliament speaker Ali Larijani over the restructuring of ministries, reports said Thursday, in a fresh sign of tension in Iran's ruling conservative camp.

The row comes shortly after an unprecedented rift surfaced between Ahmadinejad and supreme leader Ayatollah Ali Khamenei which saw the president disappear from public life for nearly two weeks.

The European Union may consider extending sanctions against Syria to include President Bashar Assad himself, EU diplomacy chief Catherine Ashton said Thursday.

"President Assad is not on the list but that does not mean the foreign ministers won't return to this subject," Ashton told Austrian Oe1 public radio in an interview broadcast on Thursday.

U.S. President Barack Obama's long-expected speech on turmoil in the Arab world and North Africa could take place as soon as next week, in the aftermath of the death of Osama bin Laden.

Obama's address will focus on the popular uprisings, violence and political change sweeping the region, and also come at a time of deep uncertainty for his hopes of renewing talks between Israel and the Palestinians.