US Defense Secretary Chuck Hagel has sent America’s top general in Europe back early from a trip to Washington in what a spokesman yesterday called a prudent step given Russia’s “lack of transparency” about troop movements across the border with Ukraine.

General Philip Breedlove, who is both Nato’s Supreme Allied Commander Europe and the head of the US military’s European Command, had been due to testify before Congress this week.

Instead, he arrived in Europe on Saturday evening and will be consulting with allies.

“(Hagel) considered Breedlove’s early return the prudent thing to do, given the lack of transparency and intent from Russian leadership about their military movements across the border,” Rear Admiral John Kirby, a Pentagon spokesman, told Reuters.

US officials, speaking last week on condition of anonymity, expressed deep concern about the massing of what they estimate are up to 40,000 Russian troops on Ukraine’s border.

That is stoking concerns in Washington and elsewhere that Russia is preparing a wider incursion into Ukraine after its annexation of Crimea.

The US and EU have meted out two rounds of sanctions on Russia, including visa bans and asset freezes for some of Putin’s inner circle, to punish Moscow, and they have threatened further actions.

The US military has also taken steps to reassure Nato allies, increasing the number of its aircraft in regular Nato air patrols over the Baltics and beefing up a previously planned training exercise with the Polish air force. More steps are being considered.

Breedlove will meet Nato foreign ministers at their April 1-2 conference in Brussels.

“The general’s return will allow him more time to confer closely with his staff and our allies and partners, and to better advise senior leaders,” Kirby said.

Hagel made the decision to send Breedlove back to Europe on Friday night, he said.