Vladimir Putin urged Ukraine to join forces with Russia Saturday during a visit to Kiev which saw feminist group Femen claim three of its activists were "abducted" ahead of a protest.
The Russian president said the inhabitants of the two former Soviet states were "one people", highlighting an ongoing tug-of-war Moscow has with Brussels over Ukraine's moves to seek closer ties with the EU.
In Ukraine to celebrate the 1025th anniversary of the arrival of Christianity in what was once known as Kievan Rus, a vast territory comprising modern-day Ukraine and Russia, Putin said the two majority Orthodox neighbours should further integrate economically.
"Intense competition is going on now in global markets, for global markets," Putin said after talks with Ukrainian President Viktor Yanukovych.
"Only by joining forces can we be competitive and win in this rather tough competitive fight."
"We have every reason to believe that we can and must do it," he said.
However he added that Russia would "respect whatever choice the Ukrainian people and the Ukrainian state will make".
The religious festivities come ahead of a November summit in Vilnius that could see the European Union sign a landmark political and economic deal with Ukraine.
This would open up EU markets for Ukrainian exports. At the same time, the Kremlin has been pushing Kiev to join a Russian-led customs union.
In talks with Yanukovych, Putin said Russia was determined to forge a closer partnership with Ukraine and the Ukrainian president acknowledged the two countries had "lots of mutual interests".
The talks were seen as a last-ditch effort by Putin to persuade Yanukovych to drop plans for closer ties with the European bloc.
The agreement with Brussels has been repeatedly postponed after a Ukrainian court jailed former prime minister Yulia Tymoshenko for seven years in 2011, triggering a dramatic deterioration in ties and making Kiev more vulnerable to the Kremlin's advances.
In a sign of continuing tensions, the European Union said Friday the "time was running out" for Ukraine to sort out a number of trade disputes if it wanted to sign the agreement.
Since coming to power in 2010, Yanukovych has performed a delicate balancing act by seeking closer European ties while also trying to remain on good terms with its prickly Soviet-era master, Moscow.
Russia has sought to keep Ukraine in its orbit but Kiev has steadfastly resisted pressure from Moscow, which wants to gain control of Ukraine's gas pipeline network.
Casting a shadow over the summit, Femen said three of its activists and their photographer had been abducted shortly before a planned protest against Putin's visit.
Femen, known for their bare-breasted protests, said the four were beaten up, bundled into a car and spirited away.
"When three Femen activists and their photographer tried to leave for the site of the planned event, they were attacked by a group of unknown assailants and beaten up," the group's leader Anna Hutsol told AFP by phone.
Hutsol also said she was hit in the face by a stranger, who made off with her dog before speeding away in a car.
"In cooperation with special services of Ukraine, special services of Russia have unleashed genuine terror against Femen activists and their backers," the group said in a statement.
Accompanied by Russian Orthodox Patriarch Kirill and Yanukovych, Putin took part in a prayer service in Kiev.
Overlooking the Dnipro River, the park where the ceremonial prayer took place hosts a monument to Prince Vladimir the Great revered for converting ancient Rus to Christianity in 988.
As well as promoting economic ties, Putin also stressed the bonds between both countries forged by a common history and what he called Russia and Ukraine's "spiritual unity".
"Together we went through great trials, tribulations and tragedies, together we built and defended the Great Rus," Putin said during a meeting with Ukraine's top Orthodox clergy.
"All of us are spiritual successors of what happened here 1025 years ago. And in this sense we are certainly one people."