Syrian security forces opened fire Friday killing 18 people as thousands of anti-regime protesters poured onto streets of flashpoint cities after the Ramadan weekly prayers, rights activists said.

Undaunted, protesters also took to the streets after the Tarawih evening prayers in several suburbs of the capital Damascus and in Homs, Deir Ezzor, Latakia, Hasakeh, Daraa and many other regions.

France on Friday condemned the arrest of the head of the Syrian League for the Defense of Human Rights and called for his immediate release.

"The arrest of Mr. Abdel Karim Rihawi... constitutes another unacceptable decision by the authorities in Damascus and goes directly against the expectations of the international community," the foreign ministry said.

U.N. leader Ban Ki-moon on Thursday expressed alarm over the rising number of civilian casualties in the Libya conflict, including those inflicted in NATO airstrikes.

Without specifically naming any side, Ban called on "all parties" to use "extreme caution" in the battle, said a U.N. statement. Ban also stepped up calls for a political solution to the conflict, in which rebels have sought to overthrow Moammar Gadhafi's four-decades-old regime.

President Dmitry Medvedev has signed a decree toughening sanctions on Libya five months after the measure was passed by the U.N. Security Council in March, the Kremlin said Friday.

It was not immediately clear why Medvedev signed the decree implementing the sanctions so long after Resolution 1973 was passed.

U.S. Secretary of State Hillary Clinton, asked why Washington has not yet explicitly called for the Syrian president’s departure, said Thursday her country wanted such a call to come from “around the world,” not just from the White House.

“What we really need to do to put the pressure on Assad is to sanction the oil and gas industry. And we want to see Europe take more steps in that direction. And we want to see China take steps with us,” Clinton added in an interview with CBS News.

U.S. President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan agreed during a phone call Thursday on the need for a "transition to democracy" in Syria, the White House said.

Obama's office said he and Erdogan had also agreed on the need for an "immediate halt of all bloodshed and violence against the Syrian people" by forces loyal to President Bashar al-Assad.

Syria's U.N. ambassador said Wednesday that anti-government groups have killed 500 Syrian security forces and sparked diplomatic fury by comparing the deadly unrest to the riots in Britain.

Britain's U.N. representative dismissed Syrian Ambassador Bashar Jaafari's "ludicrous" comparison between the handling of the worst British riots in a generation and the killing of protesters on the streets of Syrian cities. Rights groups say over 2,000 people, mainly protesters, have been killed.

Syrian forces killed at least 10 people on Thursday while the army stormed another two towns in pursuit of anti-regime protesters, defying Western calls for action after a "chilling" U.N. Security Council briefing.

The killings occurred soon after columns of tanks entered the town of Qusayr in the central province of Homs early on Thursday, sending residents fleeing into the fields, rights activist there said.

A member of Hamas' armed wing was killed Thursday in a "mysterious explosion" in the southern Gaza town of Rafah, the group and Palestinian medical sources said.

The Ezzedine al-Qassam Brigades announced the death in a statement, naming the militant as 20-year-old Ali Nayef al-Haj.

Four suspected al-Qaida members have been killed by army fire outside the southern Yemeni city of Zinjibar, most of which has fallen under the control of Islamist militants, a local official said Thursday.

The suspects were killed when their positions were hit by army shells in the villages of al-Khamla and Bajdar outside Zinjibar, the capital of the lawless Abyan province, the official told Agence France Presse requesting anonymity.