Some European countries are expected to break with Washington and start supplying the Syrian rebels with weapons in the next few months, the representative of the Syrian opposition in Britain has told the Guardian.

The National Coalition's London representative, Walid Saffour, predicted that by the next meeting of the western and Arab Friends of Syria group in Turkey, due in late spring or early summer, "there will be a breakthrough that will end the restrictions of the European countries".

"This would be for the ammunition we require, the quality weapons we need to deter the Syrian regime from using aeroplanes and Scud missiles to bomb villages and bakeries," Saffour said. "We on the ground are advancing steadily but we are suffering from a lack of ammunition. We expect that to change at the next Friends of Syria meeting in Istanbul."

Another opposition figure involved in supplying the rebels said there had been a noticeable relaxation in recent days of the strict restrictions the US and Turkey had put on arms flows over the Turkish border. He claimed a Syrian army helicopter and a Mig warplane had been shot down in the past two days, for the first time by imported missiles.

"These were not weapons that had been captured from Syrian army bases as before. These were released from the Turkish warehouses. These are weapons the opposition had purchased previously but had not been allowed to take across the border," the opposition source said.

"Before, 23mm was the maximum calibre for anti-aircraft guns permitted and we were allowed to bring in RPGs [rocket-propelled grenades] but not armour-piercing shells. But there is a major shift on the ground now. The policy is changing.

"I think the shift in American attitudes goes far beyond the official reports. I think that Washington knows it can no longer allow to let the problem fester."

The EU formally changed its arms embargo on Syria on Thursday to allow the supply of armoured vehicles, non-lethal military equipment and technical aid to the opposition. The move came as the US secretary of state, John Kerry, made his first trip to a Muslim nation since taking office, visiting Ankara, where he met Turkish leaders to discuss Syria.

While Saffour did not name the countries he expected to supply arms, the British government, which took the lead in pushing for the relaxation of the sanctions, is expected to act swiftly in reaction to the new EU rules. Foreign secretary, William Hague, is due to make a statement to parliament next week detailing the new equipment and training the UK will give the rebels. The aid is expected to include civilian vehicles – reinforced to provide protection against shelling – of a kind the British government is already supplying to UN aid workers operating in Syria.

On British insistence, the EU embargo will come up for review in June and the UK is expected to push for a further relaxation in what can be provided to the opposition if there is no let-up in the two-year-old conflict, in which more than 70,000 people are estimated to have died.

Speaking at the Friends of Syria meeting in Rome on Thursday, Hague stressed military aid was possible in the future. "That will be an important decision, of course, and has its own risks, and that is why we haven't done that so far. But I don't rule that out," he said.

A British official said: "We are going to keep on raising the pressure on the Assad regime. The Friends of Syria meeting in Rome was not the end of a process. It is the beginning of a process."

Saffour, the National Coalition's representative, said: "If the EU embargo doesn't change, then some of the EU countries will change their policy – if not openly, then quietly." He said US officials had also told the coalition that the White House policy of providing non-lethal aid only would come under review in the next few months, as new members of Barack Obama's administration, such as Kerry and the defence secretary, Chuck Hagel, enter the internal debate.

There have been reports over the past few weeks that the flow of arms to rebel fighters has increased markedly, including some anti-tank weapons made in the former Yugoslavia. The Croatian government has denied reports that its arms industry supplied some of the weapons.

Ivica Nekic, the managing director of the Croatian arms export agency, said: "We would be aware of any sale from Croatia, and no Croatian weapons have been sold to anyone in Syria."

Asked about reports that Croatian arms were supplied through Saudi Arabia, Nekic said Croatia had only sold the Saudi military helmets and was in the process of negotiating a sale of pistols. Asked about other customers in the Middle East, Nekic said: "We sell to more than 50 countries around the world. I can't speculate on every country, and every sale we make. It would not be correct."

Muhannad Hadi, the World Food Programme's regional emergency co-ordinator for Syria and neighbouring countries, said the British donation of armoured cars had already saved the lives of WFP staff delivering food aid on both sides of the lines.

"They turned out to be a life-saving tool. Our weapons were attacked four times in the past several months, by mortar. And the thing is nobody was injured."

The WFP currently supplies food to 1.5 million people in Syria in all 14 of the country's governorates, and in many parts of the country there are pockets with unknown populations which are beyond the agency's reach because of fighting. Hadi said the WFP hoped to increase the number of recipients to 2 million this month and 2.5 million next month, but was facing a critical shortage of funding.

"If we don't have funds in May for Syria, there will be serious problems. There will be breaks in the pipeline. Food needs a lead time to bring it in, with shipping and logistics, so we need the funds very, very soon."