NATO's Secretary General Anders Fogh Rasmussen has said the organisation is to hold talks with Russia on Monday.

Mr Rasmussen also said that "maybe around two thirds" of Russian troops have pulled back from the Ukrainian border.

Earlier, the Organization for Security and Co-operation in Europe said another team of observers has been detained in eastern Ukraine. 

The OSCE said the team of four international monitors and a Ukrainian language assistant was stopped by armed men in the town of Severodonetsk, 100km north of Luhansk.

It said it had lost contact with the team yesterday evening.
The detained team from the OSCE's special monitoring mission is in addition to another missing team in the east of the country, which was last heard from on the evening of 26 May, the OSCE said.

Meanwhile, Ukraine's acting defence minister has said Ukrainian forces will press ahead with a military offensive against rebels in the east until peace and order have been restored there.

Speaking after 12 servicemen, including a general, were killed yesterday when rebels shot down an army helicopter, Mykhailo Koval said: "Our given task is to bring peace and order to the region."

He repeated charges that Russia was carrying out "special operations" in the east of Ukraine.

Mr Koval said Ukrainian forces would continue with military operations in border areas "until these regions begin to live normally, until there is peace".

Earlier, the US expressed concern that pro-Russian separatists in Ukraine were using advanced weapons "from the outside".

"We are disturbed by the ongoing violence in eastern Ukraine, including reports that separatists have shot down a Ukrainian military helicopter," White House spokesman Jay Carney said.

"Now we cannot yet verify the details of these reports, but we are concerned that this indicates separatists continue to have access to advanced weaponry and other assistance from the outside," Mr Carney added.

Russia has consistently denied accusations of providing assistance to the pro-Russia separatists.

The Mi-8 helicopter gunship was shot down with a sophisticated surface-to-air missile.

Acting President Oleksander Turchynov said the weapon used was a Russian man-portable air defence system.

As well as hinting at Russian assistance, Mr Carney voiced concern for a team of Organisation for Security and Cooperation in Europe observers being held by separatists in the city of Slaviansk.

He said their detention was "unacceptable" and that they "should be released immediately".

Separatists confirmed that they detained the four observers, who have been missing since Monday.

Meanwhile, US Secretary of State John Kerry has said that Russian troops massed on Ukraine's borders are moving back towards Moscow, but there are still "danger signs".

“There is evidence of Russians crossing over, trained personnel from Chechnya trained in Russia, who've come across to stir things up, to engage in fighting," he told PBS television.

He urged Russia to take advantage of the recent presidential elections and "build a road forward where Ukraine becomes a bridge between the West and the East".

"The troops that were on the border are moving back towards Moscow not towards Kiev," he said.

"There are still danger signs there that we hope will change."

In a phone call on Wednesday, Russian Foreign Minister Sergei Lavrov had "expressed a hope that there might be a way forward," Mr Kerry said.