The Tajik government has protested after the murder of a migrant worker in Russia, in an attack which Russian nationalists say they carried out.
A group calling itself the Russian Nationalists' Combat Group sent a photo of the dead man's severed head to a local human rights organisation.
The head was later found in a rubbish bin in western Moscow.
Tajikistan, the poorest of the ex-Soviet republics, urged Russia to do more to protect its citizens.
Abdulmazhib Dostiev, the Tajik ambassador to Moscow, said he was sure that nationalists had been involved in the murder.
He told Russian newspaper Kommersant that 39 Tajiks had been murdered in Russia in the first six months of 2008.
According to the Russian news agency Interfax, about one million Tajik migrants currently work in Russia.
Murder witness
The e-mail to the Moscow Human Rights Bureau (MHRB), an EU-funded Russian civil liberties agency, was sent a few days before the head was discovered.
It threatened attacks on Moscow officials if they did not act to control immigration into the Moscow region.
The e-mail said that the victim, named as Salekh Azizov, had been killed on 6 December in a forest south of Moscow.
His head was found on Wednesday in a bin outside the district administrative building in western Moscow's Mozhaysky district.
An expert from the MHRB, Semyon Chorny, told Kommersant the site appeared to have been chosen because it was close to where a Russian teenaged girl, Anna Beshnova, was raped and murdered on 1 October.
An Uzbek migrant worker was arrested and charged with that crime, which sparked angry nationalist rallies outside the Mozhaysky administrative building.
Russian investigators say they are hoping to track down the killers of Mr Azizov, based on testimony from a fellow Tajik who witnessed his murder.
They are also investigating the source of the e-mail.
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