The Organization for Security and Cooperation in Europe lost contact Friday with a five-member team in east Ukraine, where four members of another OSCE mission are still being held by pro-Russian rebels.

The OSCE said in a statement published online that it lost contact with the team, made up of four international workers and one Ukrainian translator, in the Luhansk region late on Thursday. The OSCE has been out of contact with another four-member team in the neighboring region of Donetsk since Monday.

An insurgent leader in the Donetsk region confirmed Thursday that members of the first group were in their custody. The rebels told journalists that they would "deal with this and then release them," but didn't elaborate or give a specific timeframe. The OSCE's teams are in Ukraine to monitor the security situation following Russia's annexation of Crimea and the rise of the pro-Russia separatist insurgency in the east.

Ukraine has waged an intensifying campaign against the pro-Moscow rebels, who have seized government and police buildings and want the region to join Russia. President-elect Petro Poroshenko, who will be sworn in to office on June 7, has promised that the campaign against the rebels, who he compared to Somali pirates, would last "hours" rather than months.

Ukraine's acting defense minister, Mykhailo Koval, said Friday that the government would bring "peace and order" to the region and would not end its anti-terrorist operation "until Ukrainian authority is re-established there."

But the rebels have dug in their heels and waged an aggressive campaign against the ill-equipped army, which has little experience combatting elusive guerrilla militia groups. On Thursday, rebels shot down a helicopter carrying Ukrainian troops, killing at least 11 soldiers and a general who was in charge of combat training.