Britain summoned Syria’s ambassador Friday in coordination with other EU nations to warn of fresh sanctions against the regime if it fails to stop a crackdown on protesters, as Turkey's premier said it was premature to say whether President Bashar al-Assad should quit.

In a statement, the Foreign Office said it had "called in the Syrian Ambassador Dr. Sami Khiyami ... to express the UK’s strong concerns about the ongoing situation in Syria."

The office of the U.N. human rights chief said on Friday that it was extremely worried about reports that 700 to 850 people have been killed in Syria's pro-democracy protests since March.

A spokesman for U.N. High Commissioner for Human Rights Navi Pillay again urged Syrian authorities to halt its bid to "silence opponents".

Thousands of people rallied in Cairo's Tahrir Square on Friday calling for unity and the rule of law to be enforced after attacks on churches, state television reported.

Footage aired from the iconic square, the epicenter of protests that overthrew president Hosni Mubarak in February, showed thousands of protesters waving flags and banners in support of national unity.

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.