BRUSSELS (Reuters) - Belgium has shortened the maximum length of visas for Congolese diplomats, in a sign of concern over the political crisis in its former colony.

A spokesperson for Belgium's foreign ministry said it had limited the maximum visa duration for holders of Congolese diplomatic passports from one year to six months in response to "the overall situation" in Democratic Republic of Congo.

At least 50 people were killed last month in clashes between security forces and protesters angered by what opposition groups say are efforts by President Joseph Kabila to delay a presidential vote in order to cling to power beyond the end of his mandate in December.

The electoral commission said on Saturday that it expects the election, originally scheduled for November, to take place in December 2018.

The country's highest court has ruled that Kabila can remain in office until a new president is elected.

Kabila, who has ruled Africa's top copper producer since his father's assassination in 2001, is barred by the constitution from standing in the next presidential election.

The United States has already sanctioned three members of Kabila's inner circle for allegedly committing human rights abuses and blocking the democratic process. It has threatened further sanctions.

European Union countries have yet to impose formal sanctions, fearing that such a move could be counter-productive and lose them influence with the government.

Some have escalated their rhetoric since last month's clashes, however. On Tuesday, the French foreign ministry said that the EU should consider imposing sanctions on those responsible for human rights violations.

Congo's government denies accusations of human rights abuse and has said that sanctions represent an infringement on national sovereignty and a form of neo-colonialism.

(Reporting by Robert-Jan Bartunek; Writing by Aaron Ross; Editing by Tim Cocks and Hugh Lawson)