A pro-Western opposition party says it is challenging a key gas and oil deal with Russia at Serbia's Constitutional Court.

The Liberal Democratic Party says in a statement that the deal was unconstitutional because it includes the sale of Serbia's state oil monopoly without a public bidding and gives away strategic resources to another country.

The party filed a formal petition to the Constitutional Court on Tuesday.

The agreement was signed in January and overwhelmingly approved in the Serbian parliament last week.

It envisages that part of a pan-European gas pipeline will run through Serbia while the country sells the majority stake of the state oil company, NIS, to the Russians.