JUBA (Reuters) - A senior U.S. Federal Bureau of Investigation official will travel to South Sudan to help investigate the killing of a journalist critical of the government, the U.S. embassy said on Tuesday.

Diing Chan Awuol, who wrote online opinion pieces for newspapers and blogs, was shot in the face at his home in Juba this month, police said.

Journalists frequently complain of harassment and detention by South Sudan's security services, but it was the first time one had been killed since the country became independent last year.

On December 13, South Sudan's President Salva Kiir accepted an offer from the U.S. ambassador of FBI assistance in finding the killer, the U.S. embassy said in a statement. The official will arrive this week, it added.

A week before his death, Awuol - who wrote under the pen name Isaiah Abraham - had complained unknown men were trying to silence him, his brother said this month.

South Sudan seceded from Sudan under the terms of a 2005 peace deal that ended decades of civil war. The long conflict left the new nation severely underdeveloped and awash with weapons.

France-based Reporters Without Borders ranks South Sudan 111th out of 179th in its 2011-2012 press freedom index.

(Writing by Alexander Dziadosz; editing by Andrew Roche)