The Iraqi Prime Minister told America that it could not afford to lose the war in his country as he sought to win over US politicians sceptical of his leadership and unhappy about his forthright criticism of Israel's bombing of Lebanon.

"If democracy is allowed to fail in Iraq and terrorists are allowed to claim victory, you will never achieve victory in the war against terrorism in other parts of the world," Nouri al-Maliki told a joint meeting of the US Congress this afternoon.

"Iraq is the battle that will determine the war."

Several leading Democratic senators had threatened to boycott his speech, demanding an apology for his comments about Israel's war with Hezbollah, which he has callled "criminal".

Mr al-Maliki spoke of his detemination to rebuild Iraq and to abolish the Sunni and Shia militias that are tearing the country apart. He said that Iraq was making progress in the struggle against religious extremists, whom he accused of perverting Islam.

"Let me be very clear," he said. "This is a battle between true Islam, for which a person’s liberty and rights constitute essential cornerstones, and terrorism, which wraps itself in a fake Islamic cloak."

The speech was the second major engagement of Mr al-Maliki's trip, which has been overshadowed by the crisis in the Middle East.

Yesterday, the Iraqi Prime Minister was assured of the support of President Bush and promised more US soldiers to address the breakdown of order in Baghdad, but the positive tone of the meeting was undermined by the two men's differences over Israel's war against Hezbollah.

Although Mr al-Maliki has softened his condemnation of Israel, his language when asked about the conflict has been markedly different to House and Senate resolutions in support of Israel that passed overwhelmingly in Congress last week. Today, Iraq sent $35 million (£19 million) of aid to Lebanon to address the worsening humanitarian situation.

A group of around 20 Democrat senators and representatives wrote to the Republican Speaker of the House, Dennis Hastert, yesterday, saying that Mr al-Maliki should not be allowed to address Congress until he retracted his remarks about Israel.

"We are unaware of any prior instance where a world leader who worked against the interests of the United States was afforded such an honour," the letter said. "We believe it is essential that you clarify your position with respect to Hezbollah and its provocative incursions into Israel."

Although Republicans, including the Senate Majority Leader, Bill Frist of Tennessee, have distanced themselves from the Democrats' complaints, some emphasised their concern today that Mr al-Maliki, who was appointed to lead Iraq's government of national unity in May, is the right man to save the country from sectarian conflict.

"I certainly hope that he stands as strong as he can because we’ve got an awful lot of the credibility of the United States riding on his ability to lead this government," said Republican Senator John Warner, chairman of the Senate Armed Services Committee.

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