TEHRAN (Reuters) - Iran said on Monday the United States was failing in Afghanistan and that a new approach was needed, four days after Washington said it would invite Tehran to an international conference to discuss its neighbor.

Foreign Minister Manouchehr Mottaki did not say whether Iran would accept the U.S. invitation to this month's planned meeting on Afghanistan, a swift overture toward Tehran by the new administration of U.S. President Barack Obama.

Iran's government spokesman said on Saturday the Islamic Republic would consider such an invitation and that it was ready to help Afghanistan as it battles a growing Taliban insurgency.

Iran and the United States have not had diplomatic ties for three decades and are now embroiled in a dispute over Tehran's nuclear program, which the West suspects is aimed at making bombs. Iran says it is for peaceful power purposes.

But the two foes share an interest in ensuring a stable Afghanistan, analysts say.

Mottaki said the United States came to Afghanistan saying it would root out extremism, restore security and fight drugs.

All indicators in regard to these three areas show that the conditions have deteriorated sharply, he told state television.

Washington had indicated it needed a new direction but Tehran did not know how U.S. policies in Afghanistan would change, he said.

Obama, in a major shift in U.S. policy, has said the United States wants to engage Iran. The Afghanistan invitation would be the start of diplomatic approach to the Islamic Republic.

IRAN ROLE

While Iran and the United States sat at the same table to discuss Afghanistan after the September 11, 2001, attacks on the United States, the Bush administration made sure the new pro-Western Afghan government kept Tehran at arm's length.

Analysts have predicted the Obama team was likely to begin a dialogue with Iran on issues such as Afghanistan, where the United States is trying to turn around a war it risks losing and where it plans to send an additional 17,000 troops.

Afghanistan's foreign minister made clear he wanted Iran to attend the conference, possibly on March 31, which would also bring in neighbors including Pakistan as well as other players.

We warmly welcome any precious role that Iran wants to play to strengthen peace in Afghanistan, Afghan Foreign Minister Rangeen Dadfar Spanta told reporters during a visit to Tehran.

Mohammad Hassan Khani, an assistant professor of international relations at Tehran's Imam Sadiq University, said it was in the interests of both Islamic nations and the West that Tehran and Washington tried to resolve their disputes.

Efforts that include goodwill, engagement and negotiation certainly should be more successful than continued hostility, he wrote in the Washington Post. The United States has made a start by including Iran in discussions regarding Afghanistan.

Despite the U.S. offer to talk to Iran if it unclenches its fist, Iranian media on Sunday said the Islamic state had tested a new surface-to-air missile to help it confront any threats.

(Additional reporting by Parisa Hafezi; Writing by Fredrik Dahl; Editing by Angus MacSwan)

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