Russian President Vladimir Putin on Thursday banned the release of information about troop deaths in "special operations" during peacetime, as the Kremlin continues to rebuff accusations its soldiers are fighting in Ukraine.

Putin signed an amendment adding to what Russia classifies as "state secrets" any "information revealing losses of personnel... in peacetime during special operations."

The addition means that those who disclose details about the deaths of soldiers sent on operations could now be prosecuted.

The move drew strong condemnation from Washington, which again accused Moscow of trying to hide its direct involvement in the Ukraine conflict.

"We see this as a misplaced effort to cover up what everyone knows and that is that Russian active-duty military personnel are fighting and dying in eastern Ukraine and that the Russian government is denying it," State Department spokesman Jeff Rathke said.

Analysts said the legal change was aimed at stamping out any leaks on military losses in Ukraine after activists released a string of evidence pointing to the deaths of Russian government soldiers across the border in the former Soviet state.

"The reason is not to reveal losses in Donbass," said military analyst Pavel Felgenhauer, referring to the war zone in eastern Ukraine, adding that the legal move was designed to "imprison or scare people".

"I've never heard a legal definition of the concept of a special operation," said Felgenhauer. "That means you could classify anything you like as this."

Kremlin spokesman Dmitry Peskov said the amendment was "not linked to Ukraine" but rather was an improvement of Russian laws concerning state secrets, according to RIA-Novosti news agency.

Peskov added Putin has no intention of authorising special operations in Ukraine.

The decree gives no details of what exactly is meant by a "special operation". Revealing state secrets, when it does not involve handing them to a foreign state, is a criminal offence that can be punished by up to four years in jail.

- Troops mass on border -

The Kremlin decree comes after Kiev earlier this month captured two men it said were Russian soldiers fighting in separatist eastern Ukraine. It also follows a spike in reports pointing to a massing of Russian equipment close to the Ukraine border.

An AFP photographer in recent days saw Russian military vehicles loaded on a freight train in the southern Rostov region, some 20 kilometres (12 miles) from the Ukrainian border.

The photographer also saw Russian military vehicles at a training camp some 50 kilometres from the border.

The Kremlin has consistently denied government soldiers are fighting in Ukraine and claimed those fighting alongside rebels are there as volunteers.

Ukraine and the West insist Russia has not only armed and equipped the rebels but also sent in troops to spearhead fighting against Kiev's forces.

NATO has said rebels are using a lull in the fighting due to a shaky truce to bolster their forces ahead of possible new offensive.

Russian Foreign Minister Sergei Lavrov spoke by phone with his US counterpart John Kerry on Thursday, Moscow said.

They discussed efforts "to overcome the Ukraine crisis and establish a direct dialogue between the (Ukraine) conflict parties," the Russian foreign ministry said in a statement.

The presence of Russian troops in east Ukraine has been backed by reports from activists of hushed-up funerals, interviews with soldiers in the Russian media, and information gathered from relatives and social networking sites.

US-based think tank Atlantic Council released a report Wednesday detailing "irrefutable evidence of direct Russian military involvement in eastern Ukraine" that researchers gathered from open sources.

- Outrage -

The tightening of the state secrets act prompted an outraged reaction from Kremlin critics.

"Now the activity of looking for victims of 'special operations' in Ukraine will be a breach of state secrets," warned opposition leader and lawyer Alexei Navalny on his blog.

"Let the soldiers die and their relatives keep quiet. Those who don't agree, we'll jail for espionage."

Rights lawyer Nikolai Polozov wrote on Twitter: "If you want, you needn't bother giving relatives an insurance payout and a pension, and they won't tell anyone."

According to the latest survey by independent polling agency Levada published on May 5, only 20 percent of Russians believe government forces are fighting in Ukraine.

Forty-eight percent said they believed Russians were serving there as volunteers.

photo-or-am/del/mfp