UNITED NATIONS (Reuters) - The U.N. Security Council has rescheduled for mid-May a trip to the Democratic Republic of Congo called off last week amid travel chaos caused by ash from an Icelandic volcano, diplomats said on Friday.

The trip to discuss with Congo's government the future of the U.N. peacekeeping force in the central African country, originally set for April 17-20, will now take place May 14-16, a diplomat said following Security Council discussions.

Although the ash cloud was the official reason given for the postponement, several diplomats said last week that intensifying talks on a fourth round of sanctions against Iran over its nuclear program also played a role.

The United States is keen to get a sanctions resolution through the council by early May.

Council members plan to meet in Kinshasa with President Joseph Kabila, who has been pressing for a swift withdrawal of U.N. peacekeepers from Congo with the approach of the 50th anniversary of independence this year and elections in 2011.

Kabila wants the Congo peacekeeping mission, known as MONUC, to start withdrawing within months and the last blue helmet out in 2011.

U.N. Secretary-General Ban Ki-moon has proposed a slower three-year phased withdrawal.

Japan's U.N. Ambassador Yukio Takasu said the council wants to visit Congo before MONUC's current mandate expires at the end of May.

Council members hope to press Kabila to allow a more gradual exit of MONUC, which diplomats and U.N. peacekeeping officials say is vital to maintaining peace in the country's turbulent east.

Since its establishment in 1999, MONUC has become the world body's largest force with 22,000 troops and police, and assumed many of the responsibilities of the Congolese state, which was torn apart by a 1998-2003 war that killed millions.

But local and Rwandan Hutu rebels still roam much of the two Kivu provinces in the east. Ugandan rebels continue to wage a campaign of terror in the remote northeast and a new rebellion has emerged in recent months in Equateur province.