China's likely next premier Li Keqiang and Russian strongman Vladimir Putin on Friday praised close ties between the two giant neighbours as they met amid power transitions in both countries.

"Sino-Russian relations have today reached an unprecedented level," Vice Prime Minister Li told Putin during his first visit to the country in his current capacity and his second in the past 20 years.

"Naturally, we will be jointly striving to take our relations to new heights. We are marching along a wide path, but of course this path is not always smooth or without difficulties.

"There are difficulties, but I am sure that by overcoming them we will be able to always find a solution to these problems and achieve success in our capacity as good friends, good neighbours and good partners," he said in remarks translated into Russian and released by Putin's office.

Li did not specify what difficulties he was referring to, and neither did Putin, who for his part said the two countries were "close friends."

"We do not have a single irritating element in our ties, but we have common interests," Putin said, adding that the two neighbours were working on a number of issues requiring "additional attention."

"We've learnt to do it the way close friends do it -- we're looking for compromise, and reaching it too."

Moscow and Beijing are discussing plans to pump Russian gas to China over the next three decades. A firm gas contract has so far proved elusive because of pricing disagreements.

On the sidelines of Li's visit, Alexei Miller, the chief executive of Russian energy behemoth Gazprom, met with the chairman of China's state oil producer CNPC, Jiang Jiemin, the Russian company said without elaborating.

The two firms signed a framework agreement in 2009 that could eventually see almost 70 billion cubic metres of Russian gas sent to China annually for the next 30 years.

The talks came as Putin geared up to return to the Kremlin for a historic third term on May 7.

Russia, the world's largest energy producer, and China, the world's largest energy consumer, set much store by their bilateral ties.

Putin has paid frequent visits to China, both in his earlier capacity as president and later as prime minister, since he first took power in 1999.

His first trip abroad since he announced his Kremlin comeback was to China last October, and he is set to return in June, the Russian foreign ministry said on Thursday.

Earlier in the day Li also met with outgoing President Dmitry Medvedev who is set to be appointed prime minister upon Putin's Kremlin comeback.

The Kommersant broadsheet reported in February that Putin had refused to host Li earlier due to his busy agenda ahead of parliamentary and presidential polls, putting a strain on bilateral ties.

The newspaper, citing sources in the Russian-Chinese inter-governmental commission, the Russian foreign ministry and the government, said Li had wanted to visit Russia in February.

The newspaper said Li was forced to postpone the visit when the Russian government indicated that Putin would not be available. Putin's spokesman Peskov declined to confirm the report on Friday.

A new generation of leaders must take over the reins of power in China within a year.

President Hu Jintao will end his second five-year term as party head this year, while Li is expected to take over from Prime Minister Wen Jiabao, who will resign in 2013.

China became Russia's top trading partner for the first time in 2010, and the two countries seek to nearly double trade to $100 billion by 2015 and then to $200 billion by 2020.