By Oliver Holmes BEIRUT (Reuters) - Syria and Iran joined forces on Saturday to condemn moves by Western powers to aid rebels fighting against President Bashar al-Assad, with the Syrian leader singling out Britain as a "shallow and immature" power. In an 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 would for the first time give non-lethal aid to Syrian rebels, describing the aid as a way to bolster the rebels' popular support. Britain supports increasing general assistance to the rebels and has not ruled out supplying arms at some point in the future if the situation continued to deteriorate. Iran, which backs Assad and his Shi'ite backers, threw its weight behind its ally on Saturday and said the U.S. move would 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. Syrian Foreign Minister Walid al-Moualem, speaking during a visit to Tehran, accused the United States of double standards. "I do not understand how the United States can give support to groups that kill the Syrian people," he said. Writing by Maria Golovnina in London; Additional reporting by Yeganeh Torbati in Dubai; Editing by Stephen Powell