Armed men, believed to be Russian servicemen, walk outside a Ukrainian military base in Perevalnoye, near the Crimean city of Simferopol, March 14, 2014. Vasily Fedosenko / Reuters The Pentagon says it has received assurances that Russian forces will not push further into Ukraine, despite Moscow massing thousands of troops on the border in what it describes as a military exercise.

US defense secretary Chuck Hagel spoke for an hour on Thursday with his counterpart, Russian defense minister Sergei Shoigu, in what was described as an occasionally blunt phone call.

Rear admiral John Kirby, the Pentagon press secretary, said Hagel sought the call to better understand Russian intentions in the wake of an annexation of Crimea from Ukraine, an act Washington and its Nato allies reject as illegal.

“At times it was direct,” Kirby said.

Kirby said Shoigu assured Hagel that the Russian border buildup was merely a military exercise; that the Russians have no intention of crossing the border; and that Moscow would not take additional aggressive measures against Ukraine.

“It’s the secretary’s expectation that minister Shoigu will meet his word,” Kirby told reporters at the Pentagon on Thursday.

Kirby said Hagel told Shoigu that Russian forces “bear responsibility” for any deterioration in security in Crimea or Ukraine, and Kirby noted that a Ukrainian chief warrant officer was killed Tuesday at a Ukrainian military base in Simferopol, Crimea.

With concerns in Ukraine high that the Russians will advance beyond Crimea, the new government in Kiev has appealed to Washington for direct military aid. The Pentagon is considering the provision only of what Kirby described as “non-lethal” materiel, although the Ukrainians have asked for lethal weaponry and equipment as well.

Kirby declined to specify what aid the Pentagon is considering, but said it “was on the order of medical supplies and uniform equipment”. The Pentagon was examining the request alongside other government agencies on what Kirby called an “expeditious” timeline.

Hagel’s call with Shoigu came after the Obama administration placed additional economic sanctions on supporters of Russian President Vladimir Putin, Russian officials and a bank with ties to the Kremlin.

Shoigu was not among the 20 Russian officials who have now been designated for asset freezes, but other members of the Russian security establishment have been, including Viktor Ozerov, the chairman of the security and defense committee of the Federation Council of the Russian Federation, and Igor Sergun, the head of Russian military intelligence and deputy chief of the Russian general staff.

Shoigu did not provide Hagel with a timetable for the Russian military exercise.

This article originally appeared on guardian.co.uk