A court in Tajikistan confiscated two Russian cargo planes on Tuesday and sentenced their pilots to jail terms for smuggling and illegally crossing state borders, in a move that Moscow said could seriously damage relations with the ex-Soviet state.

The two pilots, a Russian and an Estonian, were arrested in March after making an unscheduled refuelling stop in Tajikistan on the way back from neighbouring Afghanistan, where they had been contracted to work with humanitarian organisations.

Their lawyer, Gulom Boboyev, said authorities in Tajikistan had charged both men with smuggling a faulty engine that had been carried on board one of the two Antonov An-72 planes operated by private Russian carrier Rolkan Investments Ltd.

The court found my clients guilty of smuggling and of illegally crossing state borders, Boboyev told Reuters.

The court in Kurgan-Tyube, a town 80 km (50 miles) south of the capital Dushanbe, sentenced both Russian citizen Vladimir Sadovnichy and Estonian citizen Alexei Rudenko to eight-and-a-half years in prison. They plan to appeal the verdict.

Russia's Foreign Ministry warned of serious damage to relations with Tajikistan. It said the verdict and sentences were extremely severe and politically biased.

In a statement, the Russian ministry said: The indictment was constructed on speculation and unfounded assumptions. The Tajik side is openly violating international norms.

Tajikistan's Foreign Ministry declined to comment on the case. Neither the court nor the ministry gave an explanation for the severity of the sentences.

Moscow and Dushanbe have held talks over the possible deployment of up to 3,000 Russian border guards to protect the frontier as the 2014 deadline for the withdrawal of NATO troops from Afghanistan approaches.

Russian border guard troops withdrew from Tajikistan in 2005, ending a Soviet-era legacy and handing all power over to local authorities.

Tajikistan and Russia share concerns about drug trafficking and a spillover of Islamist militancy along the 1,340-km (840-mile) border between the Central Asian state and Afghanistan.

Moscow retains a military presence in Tajikistan and stations between 5,000 and 6,000 military troops in its 201st military base on the country's western border, but the troops are not responsible for reinforcing the country's borders.

Tajikistan, the poorest of the 15 former Soviet republics, depends heavily on the remittances sent home from around 1 million migrant workers, most of whom live in Russia.

(Writing by Robin Paxton)