A group linked to Al-Qaeda group posted the footage on the Internet on June 25, saying it killed the hostages because Russia failed to pull its troops out of Chechnya.
The four Russians were kidnapped in Baghdad on June 3. The video shows three of them being executed. An accompanying statement said the fourth Russian had also been killed.
Lavrov said experts were studying the images carefully. But he also said the footage did not seem to be "fresh" and that it was not absolutely certain that it was the Russians who were shown on it.
The hostage takers had given Russia a deadline to withdraw its forces from Chechnya and free Muslim prisoners from Russian jails.
(compiled from agency reports)
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