President Vladimir Putin's spokesman said Friday the Kremlin was doing all it could to secure the release of two men that Ukraine says are captured Russian soldiers.

"These are Russian citizens who are in captivity," Putin's spokesman Dmitry Peskov told journalists, quoted by TASS state news agency.

Two Russians captured by Ukrainian forces during a firefight in the ex-Soviet state's separatist east have admitted to serving in the Kremlin's armed forces, the OSCE said on Thursday.

"Both individuals claimed that they were members of a unit of the Armed Forces of the Russian Federation. They claimed that they were on a reconnaissance mission. They were armed but had no orders to attack," the Organization for Security and Co-operation in Europe reported after conducting interviews with the two wounded men in a Kiev hospital.

Three members of a Danish human rights group faced possible deportation Thursday after being accused of breaching immigration rules, a Russian rights activist told AFP Thursday.

The Danish, German and Latvian citizens were participating in a workshop jointly organized by the prominent Russian rights group Committee Against Torture and the Danish Institute Against Torture.

Iraqi Prime Minister Haider al-Abadi on Thursday urged the Russian leadership to boost its involvement in the fight against the Islamic State group, with Moscow pledging to supply weapons.

On a visit to Moscow, Abadi warned Russian President Vladimir Putin and Prime Minister Dmitry Medvedev that terrorism is spreading, days after IS fighters seized the strategic Iraqi city of Ramadi, dealing a major blow to the government.

German Chancellor Angela Merkel said Thursday a return of Russia to the Group of Seven major industrialized nations is "unimaginable" as long as it flouts international law in Ukraine.

"As long as Russia does not commit itself, and act according to, the fundamental values of international law, a return to the G8 format is unimaginable for us," Merkel, who hosts a G7 summit next month, told parliament.

Ukraine has charged two suspected Russian soldiers who were captured during a gun battle in the separatist eastern region of Lugansk with involvement in "terrorist activity", a security official said Wednesday.

Ukrainian Security Service spokesman Markiyan Lubkivsky wrote on Facebook that Captain Yevgeny Yerofeyev and Sergeant Aleksander Aleksandrov, now convalescing in a Kiev military hospital, were also allowed to telephone their relatives in Russia.

Ukrainian President Petro Poroshenko sparked fresh Kremlin fury on Wednesday by warning that his crisis-torn country was fighting a "real war" against Russian aggressors that could escalate at any time.

The pro-Western leader said the weekend capture of two purported Russian special forces members proved that the separatist uprising in the industrial east of Ukraine was a guise for a Moscow-orchestrated campaign aimed at breaking up the ex-Soviet state.

The United Nations and the United States condemned Tuesday a mortar attack on the Russian embassy in Damascus that did not, however, cause any known injuries.

Lithuanian Ambassador Raimonda Murmokaite, whose country holds the rotating U.N. Security council presidency this month, said the "terrorist" attack caused "serious damage."

Ministers from 47 European nations agreed for the first time Tuesday to harmonize their laws to stop so-called foreign fighters travelling to Iraq and Syria to join jihadist groups.

The agreement by the Council of Europe, a pan-European rights group which includes the EU but also takes in countries such as Russia and Turkey, will make cross-border tracking and prosecutions easier.

A chemical linked to a rare poison was found in the body of a wealthy Russian businessman who died suddenly in Britain three years ago, a newspaper report said Tuesday.

Alexander Perepilichny was helping an investigation into an alleged money laundering scheme involving Russian tax officials at the time of his death, The Times added.