The U.S. ambassador in Albania has warned that Washington will take action against politicians who do not vote a judicial reform agreement, considered key to convincing the European Union to launch membership negotiations with the Balkan country.

Donald Lu said Tuesday that the actions would be "grave and long-term ones." He did not elaborate.

He accused Lulzim Basha, leader of the opposition Democratic Party, of continually changing objections to a draft agreement.

The foreign committee of the European Parliament has strongly urged Albanian politicians to endorse the package on Thursday, and so has Johannes Hahn, the bloc's enlargement commissioner.

Albania was granted EU candidate status in 2014.

The ruling Socialists need support from some opposition members in the 140-seat Parliament to approve the reforms.