Britain has no current plans to support rebels inside Syria, the foreign secretary, William Hague, has said.

His comments came despite indications that the US is considering doing so. The US defence secretary, Leon Panetta, said on Wednesday that the Obama administration was considering "an array of non-lethal assistance" to rebels trying to overthrow the Syrian president, Bashar al-Assad.

However, in his evidence to the foreign affairs select committee, Hague said Britain would only continue to supply non-lethal assistance to "peaceful" opposition outside Syria.

"I haven't ruled out giving more non-lethal help, but we haven't countenanced doing that beyond groups that are so far located outside Syria and are trying to pursue a peaceful democratic opposition," he told MPs.

One of the constraints was concern that equipment could fall into the hands of al-Qaida, which is now reported to be operating inside the country, Hague said.

No 10 had earlier described the defection of the Syrian deputy oil minister, Abdo Hussameldin to, in his words, "join the revolution of this dignified people", as an "important moment".

Hague said negotiations in New York this week over a UN security council resolution calling for an end to the violence were "difficult" because of continued opposition from Russia and China.

Given their reluctance, he said it would not be realistic to get a stronger resolution calling for the international criminal court to try Assad and other key figures in the regime, who were compared by the committee member Sir John Stanley to the Nazi "SS extermination squads".

Russia and China were paying a "diplomatic price" throughout the Arab world for having used their vetoes to block a previous attempt to agree a resolution, Hague said.

"If our view is correct that the Assad regime cannot recover its credibility internationally or internally after spilling so much blood and that, one way or another, it is doomed, then it is in the national interest of Russia and China to support a political transition at some stage," he added.

Britain was continuing to work for a UN security council resolution based on an Arab League plan calling for Assad to step down, paving the way for the transition to a new government, he said.

"The violent overthrow of the Assad regime has unknowable consequences for that region and neighbouring countries, similarly the successful violent repression of opposition by Assad's regime leaves a totally unsustainable situation and also is a threat to peace and stability in that region," Hague explained.

He also played down the prospect of opening up international humanitarian aid corridors into Syria, saying it would require "overwhelming military force" unless the Syrian government was prepared to allow access – something it has so far refused.