NATO members Croatia and Albania have written to the alliance to raise concerns that Serbia is threatening Kosovo's security as well as stability across the wider Balkans, officials said Friday.

In a joint letter to NATO chief Jens Stoltenberg, the defence ministers of the two countries "stressed concerns over recent events which might influence the security of Kosovo but also security in a wider (regional) context," the Croatian defence ministry told AFP.

Albanian minister Mimi Kodheli and her Croatian counterpart Damir Krsticevic contacted NATO to "keep attention on maintaining Kosovo's security," the ministry said by email in response to a query.

Majority ethnic Albanian Kosovo unilaterally proclaimed independence in 2008, a decade after a conflict with Serbian forces.

Serbia still refuses to recognise the move but has taken part in European Union-sponsored talks with its former foe that are aimed at normalising ties.

However in recent weeks relations between Kosovo and Serbia, neither of them a NATO member, have reached their lowest level since the talks started in 2011.

The despatch of a train by Belgrade to Kosovo last month that was painted in the Serbian flag's colours with a sign reading "Kosovo is Serbia" in multiple languages made matters worse.

Pristina labelled it a "provocation," and stopped it from crossing the border, according to Belgrade.

NATO deployed peacekeepers in Kosovo (KFOR) after a 1999 air campaign that ousted Serbian forces fighting pro-independence ethnic Albanians. The 1998-1999 conflict claimed 13,500 lives.

A NATO official in Brussels told AFP that Stoltenberg had received the letter and said the alliance "remains committed to the security and stability of Kosovo."

"KFOR will continue its mission to preserve a safe and secure environment and to guarantee freedom of movement, for as long as necessary," the official said.

At the start of its mission KFOR had some 55,000 soldiers but currently numbers around 4,500.

Croatia, an EU member, and Albania joined NATO in 2009.