The United Nations mission in Syria will remain suspended because conflict between government and opposition forces is intensifying, a top U.N. official told the U.N. Security Council on Tuesday.

And U.N.-Arab League envoy Kofi Annan has still not secured agreement on a political transition plan that all the major powers can back so that an international meeting on the conflict can go ahead this week, diplomats said.

Russian Foreign Minister Sergei Lavrov has agreed to attend an international meeting on the Syria conflict in Geneva on Saturday, Russia's U.N. envoy said Tuesday.

U.N.-Arab League envoy Kofi Annan wants to hold the meeting of the major powers in Geneva in a final bid to get agreement on a political transition plan for Syria.

Russia on Tuesday said its regional ally Syria's downing of a Turkish jet should not be viewed as a provocation and urged all nations to tone down their rhetoric over the incident.

"We believe it is important that the incident is not viewed as a provocation or an intentional action, and that it does not lead to destabilizing the situation," the foreign ministry said in its first comment on Friday's Phantom 4 jet downing.

NATO condemned Syria's downing of a Turkish jet as "unacceptable" and expressed "strong support and solidarity" with Turkey after emergency consultations Tuesday.

"We consider this act to be unacceptable and condemn it in the strongest terms," said NATO chief Anders Fogh Rasmussen. "Allies have expressed their strong support and solidarity with Turkey... We will remain seized."

Turkey branded its former ally Syria "a clear and imminent threat," on Tuesday as its Prime Minister Recep Tayyip Erdogan vented his fury over the downing of a Turkish fighter jet.

In his most outspoken criticism of the Damascus regime, Erdogan vowed to retaliate against the "heinous act" and promised a change of military attitude to any Syrian officer approaching the common border.

The U.N. expert tasked with investigating human rights abuses in Syria has finally managed to enter the country, the U.N. said on Tuesday.

Speaking to reporters at a press conference in Geneva, U.N. spokeswoman Corinne Momal-Vanian confirmed that Paolo Sergio Pinheiro had visited Syria, confirming recent media reports.

Iran on Tuesday offered to use its good ties with Damascus and Ankara to help resolve the row between the two countries over Syria's downing of a Turkish warplane.

Syria's shooting down of the jet last Friday was "a very sensitive issue" that also concerns Tehran, Iran's foreign ministry spokesman said, just ahead of an emergency NATO meeting on the incident.

Rebel forces and Syrian army units engaged in deadly combat around elite Republican Guard posts in the suburbs of Damascus on Tuesday, as 86 people were killed across the country, a monitoring group said.

The Syrian Observatory for Human Rights said the death toll consisted of 50 civilians, 32 soldiers and four rebels.

Israeli officials asked visiting Russian President Vladimir Putin to exert efforts to stop Syrian biological and chemical weapons from falling into the hands of Hizbullah, a senior Israeli diplomatic source said.

"We asked Putin for Russia to work more actively to preserve stability in Syria, to prevent biological and chemical weapons from falling into the hands of Hizbullah or other terror groups," the source told Israeli daily Haaretz.

Turkey has told the U.N. Security Council that Syria's downing of one of its fighter jets poses a "serious threat to peace and security” after Damascus insisted the plane had violated its airspace.

Turkey, in a letter to the United Nations Security Council and U.N. chief Ban Ki-moon, said Monday its plane had gone down in international airspace.