The UK has appealed for all remaining hostages held by armed groups in Ukraine to be freed after observers from the Organisation for Security and Co-operation in Europe (OSCE) were released by their captors.
Insurgents in Slovyansk, the epicentre of the trouble in eastern Ukraine, seized the multinational OSCE military observer team on April 25.
Foreign Secretary William Hague appealed for calm after a series of bloody clashes in eastern Ukraine and the Black Sea port Odessa.
At least 42 people died in clashes between government supporters and opponents in Odessa, south Ukraine.
The incident began with street fighting between the two sides in which at least three people were reportedly killed by gunfire, and the death toll mounted when opponents to the Kiev government took refuge in a building that caught fire after protesters threw firebombs inside.
At least 36 people were killed in the fire, according to the emergencies ministry. An interior ministry statement gave the overall death toll for the day at 42, but did not give a breakdown.
There were other deaths in eastern Ukraine after the Ukraine military launched an offensive against pro-Russian militia in Slovyansk.
Mr Hague said: "I deplore the senseless loss of life in Odessa yesterday and call on all parties to work to restore calm and law and order across the country.
"We look to the Ukrainian authorities to investigate these events and seek to bring to justice those responsible for them.
"It is important that there is an immediate de-escalation of tension in eastern Ukraine. At this critical moment, those responsible for provoking instability, particularly in Russia, must step back and allow the people of Ukraine to prepare for free and fair elections and repair their severely damaged economy. This will be the focus of my talks in Kiev next week.
"I welcome the release of the illegally detained OSCE inspectors in eastern Ukraine. This is a positive step, but it needs to be followed by the immediate release of all remaining hostages held by illegal armed groups in eastern Ukraine."
Russian President Vladimir Putin's spokesman today decried the Odessa deaths as evidence that the interim government in Kiev, which came to power following the toppling of the pro-Russia president after months of protests, encourages nationalist extremists.
"Their arms are up to their elbows in blood," Russian news agencies quoted Dmitry Peskov as saying.