The Organization for Security and Cooperation in Europe said on Tuesday it had lost contact with a four-member team in the restive Ukrainian city of Donetsk.

Their disappearance comes more than a month after another OSCE team of military observers was captured by pro-Moscow rebels in eastern Ukraine and held in the flashpoint city of Slavyansk for over a week.

"The team was on a routine patrol east of Donetsk when contact was lost (on Monday evening)," the Vienna-based OSCE said in a statement about the latest incident.

"We have been unable to re-establish communication until now."

It said the four were international members of its Special Monitoring Mission but did not disclose their nationalities.