White House spokesman Tony Snow said the United States was "deeply saddened" by what he called the "vicious murder" of the four men who were abducted by a militant coalition connected to Al-Qaeda.
A statement from UN Secretary-General Kofi Annan called the killings "heinous" and said no cause could justify them.
Annan's statement called for the immediate and unconditional release of all hostages in Iraq.
The four Russians, employees of Moscow's embassy in Baghdad, were abducted in the Iraqi capital on June 3.
The kidnappers had demanded the withdrawal of Russian forces from Chechnya.
(compiled from agency reports)
The International Coalition In Iraq
COALITION MEMBERS: In addition to the United States, 28 countries are Multi-National Force-Iraq (MNF-I) contributors as of May 31, 2006: Albania, Armenia, Australia, Azerbaijan, Bosnia-Herzegovina, Bulgaria, the Czech Republic, Denmark, El Salvador, Estonia, Georgia, Italy, Japan, Kazakhstan, Latvia, Lithuania, Macedonia, Moldova, Mongolia, the Netherlands, Poland, Portugal, Romania, Singapore, Slovakia, South Korea, Ukraine, and the United Kingdom. Fiji is participating as part of the UN mission in Iraq. Hungary, Iceland, Slovenia, and Turkey are NATO countries supporting Iraqi stability operations but are not part of MNF-I.
NON-U.S. MILITARY PERSONNEL IN IRAQ: United Kingdom, 8,000 as of May 26, 2006; South Korea, 3,237 as of May 9, 2006; Italy, 2,900 as of April 27, 2006; Poland, 900 as of May 30, 2006; Australia, 900 as of March 28, 2006; Georgia, 900 as of March 24, 2006; Romania, 860 as of April 27, 2006; Japan, 600 as of May 30, 2006; Denmark, 530 as of May 23, 2006; All others, 1,140.
(Source: The Washington-based Brooking Institution’s Iraq Index of June 15, 2006)
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