WASHINGTON (AFP) - Russia has assured the United States (US) that it will not invade Ukraine, the Defence Department said on Monday.

"Secretary of Defence (Chuck) Hagel spoke today by phone with Russian Minister of Defense Sergei Shoigu (and) Shoigu reiterated his assurance that Russian forces would not invade Ukraine," spokesman Rear Admiral John Kirby said in a statement.

Mr Kirby said the two discussed a range of issues related to the situation in Ukraine, with Mr Hagel requesting clarification of Russia's intentions in eastern Ukraine.

Mr Hagel urged an end to Russia's "destabilising influence inside Ukraine and warned that continued aggression would further isolate Russia and result in more diplomatic and economic pressure," Mr Kirby said.

Mr Hagel also "emphasised how dangerous the situation remains and expressed his desire to find a responsible way forward," and that Ukraine must be included in diplomatic talks, the US spokesman said.

Mr Hagel also asked Moscow's help in "securing the release of the seven inspectors from the Organisation for Security and Cooperation in Europe currently being held in Eastern Ukraine."

The seven observers were detained on Friday in Slavyansk by pro-Russian separatists, who have called them "prisoners of war", and want to exchange their freedom for the release of people detained by Ukrainian authorities.

Meanwhile, Moscow said that, during the phone call with Mr Hagel, Mr Shoigu called on the US to tone down its rhetoric on the Ukraine crisis.

Meanwhile, British Foreign Secretary William Hague said on Monday he will visit Ukraine as well as Georgia and Moldova next week in a show of support for Kiev and other pro-EU governments.

The announcement came as the US and the European Union stepped up pressure on Moscow, with the White House imposing sanctions on seven Russian officials and 17 firms close to President Vladimir Putin and Brussels adding 15 names to its own list.

In November, Ukraine pulled out of an agreement aimed at closer ties with Europe in favour of closer ties with Russia, sparking protests that ended the regime of pro-Moscow President Viktor Yanukovych and precipitating Russia's move to annex Crimea.