The US, Britain, and European and Arab states are to increase all aspects of support for the mainstream Syrian opposition fighting to overthrow President Bashar al-Assad, John Kerry said on Thursday.

Speaking after talks in London, the US secretary of state said he would not discuss specific weapons systems or who might supply them, though Saudi Arabia is understood to be playing the lead role in co-ordinating efforts to arm carefully vetted rebel units. The US and Britain say they provide only "non-lethal aid".

Ahmed Jarba, president of the Syrian National Coalition (SNC), had earlier urged the 11-nation Friends of Syria (FOS) group meeting at the Foreign Office to supply anti-aircraft missiles to help counter Assad's air attacks.

"Every possible avenue will be pursued by one country or another," Kerry said. "I'm not going to discuss specific weapons and what country may or may not be providing [weapons], but out of today's meeting every facet of what can be done will be ramped up, and that includes a political effort, aid to the opposition… economic efforts and sanctions."

Jarba spent last week in Washington lobbying for the delivery of Manpad portable missiles, stressing the devastating effect of Syrian government barrel bombs dropped on opposition-held areas. He spoke of trying to "convince the US to give us those weapons, or to convince them to allow our friends to provide us with those weapons". He made the case again in London. "Nobody said no, but nobody made any promises," said one diplomat present.

Jarba's spokesman, Munzer Akbik, told the Guardian "We felt a great deal of understanding for the problem and the need to find a suitable solution. Hopefuly we will find a way."

The foreign secretary, William Hague, said Britain would accord diplomatic mission status to the SNC, after a similar move by the US. He also promised increased UK humanitarian efforts with £30m of extra funding, in particular aimed at getting to areas of Syria that the UN has not yet been able to help. Kerry said the US was willing to follow Britain's example and consider funnelling aid to rebel-held areas using non-governmental organisations instead of the UN. "We are open to the idea of providing aid through any means that will get to people who need it and while a decision has not categorically been made, we are open to anything," he said.

In a short but strongly worded statement, the FOS group condemned Assad's plan for they what called "illegitimate" presidential elections next month as "a parody of democracy". Under a new "co-ordinated strategy," they pledged to "increase support for the moderate opposition National Coalition, its supreme military council and associated moderate opposition groups." Repeated use of the word moderate contrasted with the concern they expressed about "the rising forces of extremism".

Kerry also praised the bravery of British journalists Antony Lloyd and Jack Hill of the Times, who were double-crossed, kidnapped, beaten and shot by a rebel gang in the north of the country.

"This is not the first time that courageous reporters have been part of the heart-breaking story of Syria," he said. "Far too many journalists and innocent civilians have been hurt and killed or held hostage in Syria and just two days ago in Washington we met with many of the families of those who are being held in Syria.

"We are keeping up a very focused effort to try to secure their release. We reiterate our respect and our admiration to reporters who put their lives and their liberties on the line to tell the stories to the world that otherwise people would never learn."