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, rights groups told AFP on Monday.
Moscow denies that it has deployed regular troops to Ukraine to prop up separatists battling Kiev forces, but multiple indications 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 families of servicemen, said some 7,000 to 8,000 Russian troops are believed to be in Ukraine at present.
Citing her own estimates, she believes that between 10,000 and 15,000 troops have been deployed to Ukraine over the past two months.
"Unfortunately, I am convinced I am right," she told AFP, saying her calculations are based on information from families whose husbands and sons have been sent on drills close to the border, but then have gone incommunicado.
"Military commanders are conducting a secret special operation," said Melnikova, who is a member of the defence ministry's public council.
The ministry declined to comment.
NATO has said that "over 1,000 Russian troops" are in Ukraine.
Rights groups say Russian authorities have imposed a blackout on any information about the deployment of servicemen.
The Committee of Soldiers' Mothers and Citizen and the Army, another group representing servicemen, said they don't have any officially confirmed casualty lists so far.
But these and other campaigners, citing information from relatives and servicemen, said at least 200 Russian servicemen might have died in Ukraine.
Sergei Krivenko, head of Citizen and the Army, and Ella Polyakova, head of Soldiers' Mothers in Saint Petersburg, said some 100 soldiers from the 18th Infantry Brigade based in Chechnya are 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, who is a member of President Vladimir Putin's advisory council on human rights.
Separately, an opposition lawmaker, Lev Shlosberg, probing soldiers' presence in Ukraine, told AFP on Saturday some 100 paratroopers based in the northwestern town of Pskov had died.
Ukraine's defence ministry Valeriy Geletey said Russia had already "paid dearly for the invasion".
"Ukraine's black earth has become the final resting place for hundreds of Russian soldiers and officers," he said on Facebook.
Lyudmila Bogatenkova, head of Soldiers' Mothers in the southern Stavropol region, said a hospital in 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. "A large number of people are dying."
Rights groups say that while papers accompanying dead bodies specify gunshot or shrapnel wounds as a cause of death they don't 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.
"Commanders are not always bold enough to tell the truth," said Anatoly Salin, an expert for Soldiers' Mothers in the southern Astrakhan region, who fought in both Chechen wars.
"All orders are oral," added Polyakova, noting servicemen likely come under pressure from commanders and may be forced to sign papers on the non-disclosure of classified information.
Rights groups say 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.
The justice ministry last week labelled Soldiers' Mothers of Saint 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.