Up to 15,000 Russian soldiers have been sent to Ukraine over the past two months, and at least 200 may have died in combat there, according to rights groups.

Moscow denies that it has deployed regular troops to Ukraine to prop up separatists battling Kiev forces, but reports have emerged over the past weeks that Russian soldiers are on the ground in Ukraine.

Valentina Melnikova, head of the Committee of Soldiers' Mothers, the main organisation representing the troops' families, said an estimated 7,000-8,000 Russian troops were believed to be in Ukraine.

According to her estimates, between 10,000-15,000 troops have been deployed to Ukraine in recent weeks. "I am convinced I am right," she said, adding that her calculations were based on information from families whose husbands and sons were sent on drills close to the border, but had subsequently gone incommunicado.

"Military commanders are conducting a secret special operation," said Melnikova, a member of the Russian defence ministry's public council. The ministry declined to comment.

Nato has said that more than 1,000 Russian troops are in Ukraine.

Rights groups claim Russian authorities have imposed a virtual blackout on any information about the deployment of troops.

The Committee of Soldiers' Mothers and Citizen and the Army, another rights group representing servicemen, said they did not have any officially confirmed casualty lists. Other rights campaigners, citing information from relatives and military personnel, said at least 200 members of the armed forces might have died in Ukraine.

Sergei Krivenko, head of Citizen and the Army, and Ella Polyakova, who leads Soldiers' Mothers in St Petersburg, said an estimated 100 soldiers from the 18th Infantry Brigade in Chechnya were believed to have died in Ukraine.

"Authorities should say why soldiers are dying on the territory of another state and why they are keeping silent," said Polyakova, a member of President Vladimir Putin's advisory council on human rights.

Separately, an opposition legislator, Lev Shlosberg, investigating the presence of Russian soldiers in Ukraine, said on Saturday about 100 paratroopers based in the north-western town of Pskov had died.

Lyudmila Bogatenkova, head of Soldiers' Mothers in the southern Stavropol region, added: "A large number of people are dying." She said a hospital in the town of Rostov, close to the Ukrainian border, was "overflowing" with wounded soldiers. "Cargo-200 is coming from the Rostov range," she added, referring to the Russian military code for body bags.

Rights groups say that while papers accompanying dead bodies specify gunshot or shrapnel wounds as a cause of death, they do not say where they were sustained.

"These documents are astonishing. Instead of the place of death there is a blank space," Polyakova said. "We saw a similar picture in Chechnya."

She said authorities have apparently sought to keep the involvement of Russian troops under wraps and have tried not to leave a trail of evidence.

All orders are oral," she said, adding that troops had probably come under pressure from commanders and might be forced to sign papers on the non-disclosure of classified information.

Rights groups claim a pattern appears to be emerging: troops are sent on drills close to the border where they are told to change clothes and paint over identification numbers on their tanks before they are deployed to Ukraine. Their final destination appears to be a surprise to many personnel.

"Commanders are not always bold enough to tell the truth," said Anatoly Salin, who fought in both Chechen wars and now works with Soldiers' Mothers in the southern Astrakhan region.

The justice ministry last week labelled Soldiers' Mothers of St Petersburg a foreign agent, a term thick with connotations of cold war espionage.

"The Kremlin is determined to muzzle its critics and keep a strong lid on any information which suggests that Russia plays a direct part in the conflict in Ukraine, although evidence to the contrary is mounting every day," said Sergei Nikitin, director for Amnesty International in Russia.