Britain and France raised the pressure on other European Union members on Tuesday to lift a ban on supplying arms to Syria, where anti-government rebels are outgunned by forces loyal to President Bashar al-Assad.
Britain warned that it could break with the embargo altogether, which requires unanimous agreement by the EU's 27 members to take effect, while France hinted it would push to get the bloc to agree to amend the ban to allow the supply of arms.
The arms embargo is part of a package of EU sanctions on Syria that currently roll over every three months, with the last extension unanimously agreed by the EU last month and which came into effect on March 1.
Without unanimous agreement to either renew or amend the ban in three months' time, the embargo lapses.
"I hope that we can persuade our European partners, if and when a further change becomes necessary, they will agree with us," Cameron told a parliamentary committee when asked whether Britain could "veto" the embargo.
"But if we can't, then it's not out of the question we might have to do things in our own way. It's possible," he added.
Britain has not explicitly said it wants to arm the rebels, but repeatedly has said it does not rule out the option, depending on events on the ground.
In France, Foreign Minister Laurent Fabius said "a new balance of power" has to be created in Syria.
"We understand the idea of not adding weapons to weapons, but that position doesn't work in the face of reality it and that (reality) is that the opposition is bombarded by others who are getting weapons while they are not. It's a difficult position to keep," he said.
France and Britain accuse Assad of gambling on a military victory against his opponents, and hope the threat of giving the rebels arms will force him into talks and a transition of power.
After weeks of wrangling last month, Britain pushed for and won EU agreement to relax the embargo to allow non-lethal but quasi-military aid such as body armor and armored vehicles to be supplied to the rebels.
However, Britain and France say more must be done, while Germany has warned that giving the rebels with arms could lead to a proliferation of weapons in the volatile region and spark a proxy war.
Fabius said France would take steps on the embargo issue in the coming days without specifying what would be done.
According a senior French official who spoke on condition of anonymity, anti-aircraft missiles are among those weapons being considered for supply to rebel fighters in Syria.
BOLSTER THE MODERATES
Critics of the plan are alarmed at the growing number of Islamists in the rebel ranks, some affiliated to al Qaeda, and are also concerned about the ability of the Syrian National Coalition opposition group to control fighters on the ground.
Addressing concerns about weapons falling into the wrong hands, a Foreign Office official said Britain was confident of the moderate credentials of those it planned to help.
"We're talking to the opposition constantly about a whole range of support and we know who the people we want to work with are ... It's important to bolster the moderate elements of the opposition. We know who these are," the official told Reuters.
Britain, which has called the crisis in Syria a "catastrophe", has warned that the longer it continues, the more likely it is to attract radical Islamists.
The two-year-old conflict started out as pro-democracy protests, but has since descended into an increasingly sectarian war between rebels mainly from Syria's Sunni Muslim majority and forces loyal to Assad, who follows the Alawite faith derived from Shi'ite Islam.
Some 70,000 people have been killed and more than one million refugees have fled the violence.