Peace talks in Geneva between Syria's warring parties are almost certain to be postponed after further diplomatic setbacks on Friday, as Russia announced its intention to ship more weaponry to the Assad regime.

Heavy fighting continued on the ground in Syria, where it emerged that a British man and American woman had been killed, apparently while fighting with the rebels in Idlib, in the north, earlier this week.

The US and Russia had together conceived the Geneva talks between the Assad regime and the Syrian opposition, raising hopes that the two superpowers, long at odds over the civil war raging in the country, could at last make some progress in curbing the violence.

But after the Syrian National Coalition leader George Sabra ruled out taking part while civilians were being killed and "in light of Hezbollah and Iran's militia's invasion of Syria", diplomats admitted that the talks would not take place in early June as scheduled. They remain hopeful that they will go ahead, probably in July or August.

However, the US and Russia's differences were once more brought into stark relief with the news that Russia's MiG aircraft maker is finalising an agreement to ship at least 10 fighter jets to Syria. MiG's director general, Sergei Korotkov, said a Syrian delegation was in Moscow to discuss the details of a new contract for the delivery of MiG-29 M/M2 fighters.

The US has already criticised Russia for agreeing to deliver S-300 anti-aircraft missiles to Syria, a deal that has prompted alarm in neighbouring Israel.

"It is not helpful to have the S-300 transferred to the region while we are trying to organise this peace [conference] and create peace," the US secretary of state, John Kerry, said at a joint news conference with Germany's foreign minister, Guido Westerwelle, in Washington.

"We ask them again not to upset the balance within the region with respect to Israel," Kerry added. "The weaponry that is being provided … has a profoundly negative impact on the balance of interests and the stability of the region and it does put Israel at risk. It is not, in our judgment, responsible because of the size of the weapons, the nature of the weapons and what it does to the region in terms of Israel's security, so we hope that they will refrain from that in the interests of making this peace conference work."

More than 80,000 people have been killed in the fighting in Syria, according to the UN. On Friday, Syrian TV reported that the British man, Ali Almanasfi, 22, from Acton, west London, was killed, alongside an American woman and another unidentified westerner, on Wednesday. Syrian TV posted a picture of Almanasfi's passport and graphic pictures of his body were posted on the internet.

A Foreign Office spokesman said: "We understand that a British national has been killed in Syria. Their family have been informed and we are providing consular assistance."

Scotland Yard confirmed that his family had filed a missing persons report on 4 February, and it is understood the report contains the family's fears that he may have travelled abroad.

At the flat where Almanasfi's mother and sister live, his family were visited by two police officers but refused to comment on reports of his death.

Almanasfi's brother-in-law Kusai Noah said he was stunned about the news of his death. "He didn't tell anyone that he'd gone … we didn't know that he was going anywhere," he said. "He disappeared. We made a missing report for the police. This was a couple of months back."

Syrian TV identified the dead American woman as Nicole Mansfield, 33, from Michigan. "I'm just devastated," her aunt, Monica Mansfield Speelman, told Reuters. "Evidently, she was fighting with opposition forces." Speelman said the FBI had informed the family on Thursday afternoon.

Their deaths came amid growing concerns about the increasing prominence of jihadist groups within the rebel fighters. The most powerful, Jabhat al-Nusra, pledged allegiance to al-Qaida in April. The uprising against Bashar al-Assad began with peaceful protests in March 2011 but has since erupted into a bloody civil war. Assad maintained from the start that he was fighting against "terrorists", including foreign jihadists.

Almanasfi was known to MI5 along with other British citizens who have gone to Syria to fight with different rebel groups, intelligence sources made clear on Friday.

British counter-terrorist officials, almost certainly including MI5 officers, are said to have tried to persuade him not to go. "We do try and stop people from going. We have to do it by persuasion as we can't stop them from going to France or Turkey," a Whitehall source said.

Most Britons going to fight in Syria travel via France or Turkey. Between 70 and 100 are believed to have gone so far but it is difficult to be precise as nothing may be known about them before they make for Syria.

British security and intelligence officials describe the number of British citizens or residents going to fight for rebel groups, including those supporting al-Qaida, as "worrying".

The UK and France have faced criticism for forcing the EU to end an embargo on arms sales to Syrian rebels, with concerns raised that they will only prolong the conflict and inevitably fall into the hands of jihadists, despite the UK's insistence that if weapons are sent they will only go to the "good" rebels. Hours after the UK and France's actions at the EU meeting of foreign ministers on Monday, Russia announced delivery of the S-300 missiles to Syria. The deputy foreign minister, Sergei Ryabkov, argued that the delivery of the S-300 system had been previously agreed with the Syrian government in Damascus and would be a "stabilising factor" that could dissuade "some hotheads" from entering the conflict.