China said Thursday it is sending an envoy to Syria to push for a "peaceful" resolution of the conflict there, after drawing condemnation for vetoing a resolution condemning a crackdown on protesters.

Vice Foreign Minister Zhai Jun will visit Syria from February 17 to 18, foreign ministry spokesman Liu Weimin told a regular briefing.

A Syrian opposition group on Thursday rejected a newly proposed constitution, urging voters to boycott a referendum set for later this month and to step up efforts to oust President Bashar Assad.

"The Local Coordination Committees calls upon our people to reject and boycott the alleged referendum to confirm the lack of public support for this criminal regime," the opposition group said in a statement.

Israeli warplanes early Thursday carried out two raids on the north and center of the Gaza Strip following a rocket attack on southern Israel, an army spokeswoman said.

The attacks, which came after an Israeli police spokesman said three rockets had been fired from the Gaza Strip without claiming victims, were aimed at "centers of terrorist activity," the army spokeswoman said.

Seven Iranian engineers kidnapped near Syria's embattled city of Homs in late December have not been released as previously reported, and remain captives, the Iranian foreign ministry said on Thursday.

"We hope to witness the freedom of the Iranian engineers and the rest of pilgrims soon," ministry spokesman Ramin Mehmanparast was quoted as saying by the Fars news agency.

Switzerland Wednesday said it was closing its embassy in Damascus and urged its nationals to leave Syria "as quickly as possible".

The embassy closure, which follows that by the United States, was announced by Swiss Foreign Minister Didier Burkhalter and comes against a background of increasing violence inSyria where the regime is seeking to crush a nearly year-long rebellion.

Turkey urged the United Nations on Wednesday to appeal to the Syrian leadership to allow a humanitarian operation to help Syrians suffering from the months-long bloodshed.

Speaking to reporters, Foreign Minister Ahmet Davutoglu said the U.N. should step in not only on political matters "but also on humanitarian issues," vowing that Turkey would keep the Syrian crisis "on the U.N. agenda."

French Foreign Minister Alain Juppe warned Wednesday that Russia was isolating itself after its veto of a U.N. resolution on Syria and would try to convince his Russian counterpart to change tack.

Juppe said it would take "all my powers of persuasion" when he meets Russian Foreign Minister Sergei Lavrov in Vienna on Thursday.

The United States Wednesday dismissed Syria's call for a referendum on a new constitution, which critics see as a move designed to ease global outrage over the government's bloody crackdown.

"It's actually quite laughable -- it makes a mockery of the Syrian revolution," White House spokesman Jay Carney told reporters aboard Air Force One.

Britain and the Holy See on Wednesday called for an end to violence in Syria and underlined the role of Christians in the Middle East after a British delegation met with Pope Benedict XVI.

The Vatican and Britain in a joint statement appealed for "an immediate end to violence in Syria," and stressed "the need for cooperation to overcome the present crisis" at a joint press conference in the Holy See.

Russia on Wednesday angrily dismissed claims that Syrian forces have been using chemical weapons against the opposition with the help of Russian experts.

"We categorically deny these fabrications," the Russian foreign ministry said in a statement.