Pristina (AFP) - A shadowy activist group in Kosovo claimed responsibility Monday for an attack on the home of Kosovo's public broadcast chief, in protest over a controversial border deal.

"An explosive device was thrown into the backyard" of Mentor Shala, general director of Radio Television of Kosovo (RTK), late on Sunday, according to a police statement.

The incident in the capital Pristina came six days after another grenade was thrown in the courtyard of the RTK's headquarters, causing material damage.

Responsibility for both attacks was claimed by a group called "Rugovasit", a term for inhabitants of the Rugova mountains at the disputed border with Montenegro.

On Thursday, Kosovo's parliament is due to vote on a border demarcation deal with Montenegro, which opponents fear will see Kosovo lose thousands of hectares.

In an emailed statement to media, Rugovasit said the RTK under Shala had sided with the government and overlooked the opposition because it "speaks out against the demarcation".

The statement said the latest attack was "only a warning" and urged Shala to resign, otherwise "his life is in danger".

"We urge the government to take us seriously and not make the demarcation," Rugovasit said, adding there would be "victims" if their request was not met.

Police said Sunday's device, which caused no casualties, was likely a hand grenade and an investigation has been launched.

"The explosion was so powerful," Shala told reporters.

"The entire family was inside and luckily no one was hurt by this explosion."

A government press release described the incident as a "criminal act directed against media freedom in Kosovo", while the European Union office in Kosovo said it was "very concerned".

"We are worried about the cycle of violence in Kosovo becoming more and more serious," the EU statement said, calling on all political parties to work to diffuse tensions.

An agreement on the border is crucial to Kosovo's bid to win visa-free travel in the EU and further integration with the bloc.

But proposed deal has sparked tension and protests both in and outside parliament in recent months.

International press freedom watchdogs warn that journalists and media outlets in Kosovo are often threatened over their investigative reporting, but there have been no direct armed attacks against them before.