Ukraine's acting president has called for new anti-terrorist measures to protect citizens of his country after "brutally tortured" bodies were found near Slavyansk.

Oleksandr Turchynov said the body of Volodymyr Rybak, a member of Turchynov's Batkivshchyna party, was one of two found showing signs of torture.

Mr Rybak was deputy head of Horlivka city council. Ukrainian news website vesti.ua said a body thought to be his was found beside a river about 60km from the city.

The dead man was hard to identify, the website said, as it had been in water for several days. Mr Rybak went missing on April 17.

In a statement, Mr Turchynov said: "I call on the security agencies to relaunch and carry out effective anti-terrorist measures, with the aim of protecting Ukrainian citizens living in eastern Ukraine from terrorists."

A number of political leaders and journalists are reported to have been detained by pro-Russia militia who running many cities in eastern Ukraine.

The detentions prompted the OSCE, who are in Ukraine as monitors, to express deep concern.

The separatists are demanding the right to hold a referendum on the future of eastern Ukraine, similar to that which prompted the breakaway of Crimea in March.

Vice President Joe Biden arrived in Kiev on Tuesday to reiterate US support for a unified Ukraine during a meeting with MPs.

As he did so, the US said it will be sending about 600 troops to Poland and the three Baltic states to take part in exercises.

Under an agreement signed by the US, the European Union, Russia and Ukraine in Geneva last week, the separatists were supposed to disarm and relinquish control of the towns.

The US has threatened further sanctions against Russia if the separatists - who are Russian speaking and are allegedly backed by Moscow - do not meet the agreement's conditions.

Mr Biden said: "There are some who are trying to pull Ukraine apart. Ukraine is and must remain one country - one united Ukraine.

"No nation has the right to simply grab land from another nation. More provocative behaviour by Russia will lead to more costs and greater isolation."

Not long after he made his speech, a Ukrainian military surveillance plane flying over Slavyansk, in eastern Ukraine, was hit by small arms fire, forcing it to make an emergency landing.

Ukrainian Prime Minister Arseny Yatseniuk said Russian special forces were operating in eastern Ukraine and urged Russia to withdraw troops in Crimea to "turn this shameful page".

But Russia rebuffed the claims, with foreign minister Sergei Lavrov calling for the US to put pressure on Kiev to abide by their part of the Geneva accord.

On Easter Sunday, at least three people were killed at a checkpoint manned by armed separatists, with Russia blaming Ukrainian nationalist group Right Sector.

British Prime Minister David Cameron will discuss the Ukraine crisis and EU reform during a meeting with European Council President Herman Van Rompuy later on Tuesday.

The Organisation for Security and Co-operation in Europe (OSCE) has deployed around 100 mediators in Ukraine to monitor the situation in the east of the country.