The OSCE said Thursday that it has made contact with two teams of four observers who were kidnapped in eastern Ukraine three weeks ago and that they are all "unharmed".

"All the information indicates that all members in both teams are unharmed," Organization for Security and Cooperation in Europe spokeswoman Natacha Rajakovic said.

Ukraine's new pro-Western President Petro Poroshenko said on Thursday he would sign the crucial trade and economic relations portion of an historic EU pact in Brussels on June 27.

The old Kremlin-backed leadership's rejection of the EU Association Agreement in November sparked months of deadly protests that led to the February ouster of president Viktor Yanukovych.

Russia is again building up forces on the border with Ukraine, NATO Secretary-General Anders Fogh Rasmussen said on Thursday, calling it a "very regrettable step backwards".

Rasmussen said "at least a few thousand more" Russian troops had been deployed to the border.

Russia on Thursday accused the West of attempting to torpedo Moscow's latest draft resolution on Ukraine and called on the U.N. Security Council to help alleviate the humanitarian crisis in the ex-Soviet country.

"Many Western members of the Security Council continue to look for various excuses to stall for as long as possible the work on the Russian draft which essentially opens the way for a real opportunity to launch the process of peacefully resolving the situation in Ukraine," a foreign ministry spokesman said.

Ukraine's parliament on Thursday confirmed Kiev's negotiator with Moscow as foreign minister in a boost for the new president's plan to end the bloody pro-Russian insurgency gripping the separatist east.

The unanimous approval of Pavlo Klimkin came as President Petro Poroshenko prepared to host leaders from the restive rust belt as he continued to build his case for a ceasefire following 10 weeks of fighting.

Washington on Wednesday welcomed Ukrainian proposals for a unilateral ceasefire in the troubled east, but stressed Kiev needed "a partner" to help end the fighting by the pro-Russia separatist insurgency.

Ukraine's new Western-backed President Petro Poroshenko said he would order the ceasefire in the coming days, offering an amnesty for those who laid down their arms unilaterally, as well as safe corridors for Russian fighters to return home.

Pro-Russian rebels have imposed a reign of fear in eastern Ukraine, abducting scores of real and imagined opponents and subjecting them to torture, the U.N. human rights office said Wednesday.

In a fresh report by its monitors in the conflict-torn country, the U.N. office also said hundreds had perished in fighting and that the rights situation had worsened since it took stock in early May.

Ukraine's new Western-backed president announced plans Wednesday to order a unilateral ceasefire in the separatist east that could help end a bloody pro-Russian insurgency and avert his ex-Soviet country's breakup.

Petro Poroshenko took a further step toward relieving tensions with Russia by deciding to replace acting Foreign Minister Andriy Deshchytsya -- a hate figure in Moscow -- with his current envoy to ongoing OSCE-mediated negotiations with the Kremlin.

Russia said Wednesday it was investigating Ukraine's interior minister and a local governor over the killings of civilians and journalists in the conflict-torn east of the ex-Soviet country.

"A criminal probe was started against Igor Kolomoisky and Arsen Avakov," the Moscow-based Investigative Committee said, referring to the billionaire governor of the Dnipropetrovsk region and Ukraine's interior minister.

Russian President Vladimir Putin and his Ukrainian counterpart Petro Poroshenko held talks on Tuesday over a possible ceasefire in Ukraine, as two members of a Russian television crew were killed in the ex-Soviet state's separatist east.

Poroshenko assured Putin that an investigation would be launched into the deaths, and vowed to take necessary measures to protect reporters covering the conflict, the Kremlin said in a statement following the talks.