Russia has increased its military activity near the border with Ukraine markedly since late last week, a Reuters reporting team said yesterday after making return visits to the frontier zone where Nato says Moscow has amassed 40,000 troops.

Russia’s military moved equipment and weaponry around the border area yesterday.

Russia has put thousands of troops near the border for what Moscow says are routine exercises but which Nato says is an attempt to intimidate Ukraine’s Western-backed government by holding out the threat of an incursion.

US President Barack Obama on Monday called on Russian leader Vladimir Putin to pull back the forces, saying diplomatic efforts to end the crisis in Ukraine “cannot succeed in an environment of Russian military intimidation on Ukraine’s border.”

A Reuters team went independently to three locations where Russian units are temporarily deployed, first visiting at the end of last week and again yesterday. On the second visit yesterday there was a marked increase in activity, with more military trucks on the roads in the area, and more service personnel in evidence at the deployment sites.

At one location, in open country near the village of Valuyki, in Belgorod region about 20 km from the Russian-Ukrainian border new forces had arrived. The Reuters reporters saw 10 large army tents and 20 trucks, significantly more than last week. The trucks had their hoods up and soldiers were inspecting the engines.

The site was surrounded with a barbed wire fence which was not there last week.

The intervening days have seen a sharp increase in tensions across the border in Ukraine, where armed pro-Russian militants have seized buildings in about 10 towns and cities. The Kiev government has launched an “anti-terrorism” operation to retake captured towns, which Russia says could lead to civil war.