Ukraine's acting interior minister has confirmed two people have been killed and several injured in a shootout in the eastern Ukrainian city of Kharkiv.
Arsen Avakhov wrote on his official Facebook page early on Saturday that around 30 people "from both sides" were arrested late on Friday evening. Gunshots were heard in the area.
Russian state news agency Itar Tass reported that the clash occurred outside a building of the far-right Ukrainian nationalist group, Right Sector, although Avakhov made no mention of the group and said the incident was under investigation.
Both sides blamed each other for the violence overnight.
The clashes come after a six-hour meeting between the US Secretary of State and Russian Foreign Minister failed to reach an agreement on how best to end the crisis in Ukraine on Friday.
The meeting, set in the official residence of the US ambassador in central London, comes two days before a referendum on 16 March where Crimeans will decide whether the region should be claimed by Moscow, or remain in Ukraine, following Russia’s military seizure of Crimea earlier this month.
In pictures: Ukraine crisis
In pictures: Ukraine crisis
1/12 Ukraine crisis
People shout slogans during a pro Russian rally at a central square in Donetsk. Pro Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea.
2/12 Ukraine crisis
In the same pro Russian rally, demonstrators show their support. Ukraine's ambassador to Russia and a deputy Russian foreign minister held a "cordial" meeting on Saturday, Moscow said, without giving details of any discussion of Russian-occupied Crimea.
3/12 Ukraine crisis
Crimean ethnic tatars stand on the roadside as Russian troops move towards to Simferopol in the settlement of Kok-Asan, some 70 kilometres from Simferopol in Crimea.
4/12 Ukraine crisis
Russian troops stand on a roadside in the settlement of Opytnoye, some 70 kilometres from Simferopol.
5/12 Ukraine crisis
Armed members of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" march before the swearing-in ceremony in Simferopol, Ukraine. Some 30 men armed with automatic weapons and another 20 or so unarmed, were sworn in at a park in front of an eternal flame to those killed in World War II.
6/12 Ukraine crisis
A group of Cossacks march past a statue of Soviet revolutionary leader Vladimir Lenin in Simferopol as tensions in the area continue to rise.
7/12 Ukraine crisis
An armed member of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" signs the oath during the swearing-in ceremony in Simferopol,
8/12 Ukraine crisis
9/12 Ukraine crisis
Ukrainian soldiers load their armed personnel carriers (APCs) into boxcars in the western Ukrainian city of Lviv. Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on the 8th.
10/12 Ukraine crisis
An abandoned naval ship sunk by the Russian navy to block the entrance is seen in the Crimean port of Yevpatorya on March 8th.
11/12 Ukraine crisis
Ukrainian sailors stand guard on top of the Ukrainian navy ship at the Crimean port of Yevpatorya.
12/12 Ukraine crisis
Crimea's pro-Moscow leader Sergei Aksyonov speaks to the media in Simferopol on the 8th March. He has defended a decision to hold a referendum on whether the region should join Russia, saying on Saturday that "no one" could cancel the voting.
After the negotiations, John Kerry maintained the Western view that the referendum is illegitimate because of the presence of thousands of Russian troops on the Crimean streets combined with the lack of a significant campaign period.
But Sergei Lavrov said Moscow will continue to “respect” the result of Sunday’s referendum.
Mr Lavrov also rejected calls from Western powers to create an international contact group to resolve the crisis in Ukraine, citing “the crisis was not caused by Russia” and that “the relationship [between Russian and Ukraine] had never been ceased”. The UN Security Council will vote today on a US-drafted resolution that would define the Crimea referendum as illegal.
The resolution being put to a vote would reaffirm the council's commitment to Ukraine's territorial integrity and declare that tomorrow's referendum on whether Crimea should become part of Russia "can have no validity, and cannot form the basis for any alteration of the status of Crimea".
Acting president Oleksander Turchinov, who reviewed large Ukrainian military exercises on Friday, has said the massing of Russian troops showed they were "ready to invade ... at any moment".
But Mr Lavrov said on Friday that Moscow had no plans to invade southeastern Ukraine.
Additional reporting by agency