The UN-Arab League envoy to Syria has ended a visit to Damascus, calling on both the government and the opposition to attend a peace conference in Geneva later this month.
Lakhdar Brahimi, who had travelled to Damascus at the end of a Middle East tour to muster regional support for the conference, appeared uncertain about prospects for the meeting.
He acknowledged the gathering cannot take place if the opposition refuses to take part.
"We will say it's happened only when it happens," he told reporters at a press conference in Damascus, urging both sides to co-operate.
Mr Brahimi's plea came just hours after officials said Israeli warplanes had attacked a shipment of Russian missiles inside a Syrian government stronghold - a development that threatened to add another volatile layer to regional tensions from the Syrian civil war.
An Obama administration official confirmed the Israeli airstrike but provided no details. Another security official said the attack occurred late on Wednesday in the Syrian port city of Latakia and that the target was Russian-made SA-125 missiles.
There was no confirmation from Syrian officials and state media made no mention of the reports.
Since the civil war in Syria began in March 2011, Israel has carefully avoided taking sides but has struck shipments of missiles inside Syria at least twice this year.
The Syrian military, overstretched by the civil war, has not retaliated, and it was not clear whether the embattled Syrian president Bashar Assad would choose to take action this time. Assad may decide to again let the Israeli attack slide, particularly when his army has the upper hand on the battlefield inside Syria.
The US and Russia are pushing for a peace conference bringing both sides of the Syrian civil war to the table in Geneva later this month. More than 120,000 people have been killed in the nearly three-year-old conflict, according to the Syrian Observatory for Human Rights, a Britain-based watchdog that closely monitors the violence in Syria through a network of activists across the country. The UN said in July that 100,000 Syrians have been killed, and has not updated that figure since.
Millions of Syrians have been uprooted from their homes because of the fighting.
Mr Brahimi warned that if the crisis goes on, expectations are that those directly affected by the crisis may reach half of Syria's total pre-war population of 23 million people.
"It is time for Syrians to co-operate and for others in the region and outside to co-operate with them to end this crisis," Mr Brahimi said.
The envoy, who met this week with Assad and Damascus-based opposition groups, said the Syrian government has confirmed it would attend.
Deeply fractured Syrian opposition groups are also split on whether to attend the Geneva talks. They also disagree over conditions for taking part - from demands that Assad step down right away to guarantees that he would not be part of a negotiated solution for the country's future
This time, Mr Brahimi appeared to put the onus on the opposition, saying talks in Geneva cannot "go forward without the opposition".
"The participation of the opposition is essential, necessary and important," he said.