Marchuk will step down as security council chief and replace Volodymyr Shkidchenko, who was dismissed Friday for "serious failings" in his ministry's work, the presidency said.

Marchuk, 62, headed Ukraine's SBU intelligence service from 1991 to 1994, immediately following the collapse of the Soviet Union and dissolution of its KGB agency.

He also held the post of prime minister from June 1995 to May 1996 and was appointed to lead the national security and defense council in November 1999.

Marchuk was one of the Ukrainian administration's strongest supporters of the US-led war in Iraq.

As defense minister, he will now oversee the deployment of some 1,800 Ukrainian troops as part of a Polish-led stabilization force overseeing southern Iraq.

Washington has said it will largely cover the financial costs of the deployment.

Marchuk also faces the difficult task of reforming Ukraine's armed forces, which have been starved of funds and crippled by corruption since independence.

He will seek to get them in line with NATO standards in a bid to further Ukraine's attempt to win an invitation from the military alliance.

Ukraine has been attempting to reform its 390,000-strong armed forces with a budget of just 710,000 dollars (615,000 euros).

The country's military has been plagued with accidents, including the November 2001 incident when a Ukrainian missile accidentally downed a Russian passenger plane, killing 78 people.

Shkidchenko's predecessor Olexander Kuzmuk resigned after the incident.

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