Syria and Iran joined forces on Saturday to condemn moves by Western powers to aid rebels fighting against Syrian President Bashar al-Assad, with the Syrian leader singling out Britain as a "shallow and immature" power.

In a television interview with The Sunday Times newspaper shown in London late on Saturday, Assad said Britain's involvement in the Syria crisis had been naive and unrealistic.

"I think they (Britain) are working against us, and they are working against the interests of the UK itself," Assad said in English-language remarks broadcast by Britain's Sky TV.

"This government is acting in a naive, confused and unrealistic manner. If they want to play a role they have to change this, they have to act in a more reasonable and responsible way."

Assad added: "How can you ask them to play a role in making the situation better, more stable, how can we expect them to make the violence less when they want to send the military supply to the terrorist?"

Backed by the United States, Britain and much of Western Europe, Syria's opposition has made plain that Assad can play no role in a future Syrian government.

But as the situation deteriorates on the ground, the opposition is increasingly frustrated with the West's reluctance to get directly involved in the fighting, and wants foreign powers to send weapons to help its fighters.

On Thursday, the United States said it will for the first time give non-lethal aid to Syrian rebels, describing the aid as a way to bolster the rebels' popular support.

The assistance will include medical supplies, food for rebel fighters and $60 million to help the opposition provide basic services like security, education and sanitation. Britain supports increasing general assistance to the rebels and has not ruled out supplying arms at some point in the future if the situation continues to deteriorate.

Iran, which backs Assad and his Shiite backers, threw its weight behind its ally on Saturday and said the US move will only prolong the Syrian conflict, an uprising-turned-civil war in which some 70,000 people have been killed.

"If you really feel sorry about the ongoing situation in Syria you should force the opposition to sit at the negotiation table with the Syrian government and put an end to bloodshed," said Iran's foreign minister, Ali Akbar Salehi.

"Why do you encourage the opposition to continue these acts of violence?"

British Foreign Secretary William Hague said on Sunday that Assad was "delusional" for failing to see that the bloodshed in his country was on his own hands. Hague said that he will this week announce more assistance to the Syrian opposition in the form of non-lethal equipment.

Britain has been pushing to lift a ban on the sale of arms to Syria's rebels, but at a meeting last month, European Union foreign ministers ruled that only "non-lethal" aid and "technical assistance" could be given to the opposition.

Hague told BBC television: "This is a man presiding over this slaughter.

"And the message to him is we, Britain, are the people sending food and shelter and blankets to help people driven from their homes and families in his name.

"We are the people sending medical supplies to try to look after people injured and abused by the soldiers working for this man.

"Assad thinks and is told by his inner circle that all of this is an international conspiracy, not the actual rebellion and revolt of his own people.