Explosions this week at Russia's air base in Crimea, southern Ukraine, are believed to have degraded the aviation capability of the Russian Navy's Black Sea Fleet.

Massive blasts occurred on Tuesday at the Russian air field in Crimea, which Russia unilaterally annexed eight years ago.

Major US newspapers have suggested that Ukrainians were involved in the incident while Ukraine's government has not officially mentioned their involvement.

Britain's Defence Ministry said in its intelligence update on Friday that at least five Su-24 fighter bombers and three Su-30 fighter jets were "almost certainly destroyed or seriously damaged in the blasts."

The ministry also said, "The loss of eight combat jets represents a minor proportion of the overall fleet of aircraft Russia has available to support the war."

It added that the naval aviation capability of the Russian Navy's Black Sea Fleet is "now significantly degraded."

Ukraine's military said on Thursday that Russia's military aviation operations had decreased in southern Ukraine since Tuesday's explosions.

Meanwhile, former Russian President Dmitry Medvedev, who now serves as Security Council Deputy Chairman, announced on Thursday that he had visited the eastern Ukrainian region of Luhansk. Russian forces say they have taken full control of the region.

Medvedev, accompanied by aides of President Vladimir Putin, met with leaders of pro-Russian separatists in Luhansk and the neighboring region of Donetsk.

They reportedly discussed issues, including how to harmonize the two regions' laws with Russia's and reconstruction of infrastructure in the regions.

Russia appears to be preparing to hold referendums in September in the Ukrainian regions it controls. The move is said to be laying the groundwork to annex the Ukrainian territory.

Ukrainians are expected to intensify their counterattacks to retake the regions.