MOSCOW (REUTERS) - Russian border guards have stepped up checks on goods entering from Ukraine, officials from the customs services and an agricultural inspection service said on Thursday, underlining tensions between the two countries.

Ties between Ukraine and Russia are at their lowest point since the fall of the Soviet Union.

Moscow has annexed the Ukrainian region of Crimea and Kiev has threatened to retaliate, including introducing a visa regime for Russians.

"Russian customs have increased customs checks, acting on information about possible attempts to bring contraband in from Ukraine, including weapons," said Dmitry Kotikov, a customs spokesman.

Russia's Federal Service for Veterinary and Phytosanitary Surveillance said customs officers were finding more cargoes with fake documentation.

Separately, Russia's top security service said police had arrested several Russians and Ukrainians accused of smuggling weapons to Russia's volatile North Caucasus region through Ukraine.

A Ukrainian customs spokeswoman said there were no problems along the country's eastern border with Russia.

Russia last tightened customs controls on Ukraine in August last year in a move criticised as a means of piling pressure on Kiev to end negotiations on a free trade agreement with Europe.

Shunning that treaty in favour of closer ties with Russia in November prompted protests that culminated in the toppling of pro-Moscow President Viktor Yanukovich last month, ushering in a new pro-EU leadership.