Seven people have been arrested in western Ukraine in connection with a deadly overnight attack on a Roma camp in a forest on the outskirts of Lviv, police say.

Authorities say a 24-year-old Roma man from a village near the city of Rivne was killed in the attack, which was carried out by a group of masked men on June 23 just before midnight.

Police say four other people were hospitalized with knife wounds as a result of the attack -- including a 10-year-old boy, two 19-year-old men, and a 30-year-old woman.

Seven suspects aged 16 and 17 were arrested, as well as a 20-year-old accused of planning the attack, according to police.

A criminal case was opened over the "deliberate murder committed by a group of people." If convicted, the attackers could face up to 15 years in prison.

The violence was the fifth attack on a Roma camp in western Ukraine in the past two months.

In a joint letter to Kyiv authorities on June 24, four groups, including Human Rights Watch and Amnesty International, condemned what they said was a growing number of attacks by radicals.

Ukrainian authorities have "failed" to respond to most incidents, leading to "an atmosphere of near total impunity that cannot but embolden these groups to commit more attacks," the groups said.

The letter said that several neo-Nazi and far-right ultranationalist groups, including C14 and Right Sector, have been behind at least two dozen attacks or harassment cases against Roma across Ukraine so far during 2018.

The Council of Europe, an international organization that aims to uphold human rights, democracy and the rule of law, estimates that there are some 260,000 Roma in Ukraine out of a population of about 48.5 million.

With reporting by AFP