Egypt extended on Wednesday the detention of an Al Jazeera journalist for another 15 days, pending investigation.

Mahmoud Hussein, an Egyptian national, was arrested upon arrival at Cairo's airport on December 20 without charge.

Five days later, Egypt's interior ministry accused Hussein of "incitement against state institutions and broadcasting false news with the aim of spreading chaos".

Al Jazeera rejected the charges and says Hussein was in Egypt on holiday and not for work. He visits his family several times a year.

"Al Jazeera deems all accusations against Hussein, including those which might be added later to the current allegations, to be a result of practices which violate international norms and conventions, and which, unfortunately, prevail in Egypt as exposed by human rights organisations," the network said in a statement.

READ MORE: Groups call for release of Al Jazeera's Mahmoud Hussein

Hussein, who lives in the Qatari capital Doha, joined Al Jazeera in Egypt in 2011, and moved to its headquarters in Qatar in 2013.

Shortly after he was detained, state security stormed the homes of Hussein's two brothers and arrested them. They have since both been released.

Hussein has complained of bad treatment in detention, of not being allowed to contact his lawyer, of being denied visits from family members, and of being held in solitary detention, Al Jazeera has learnt.

Egyptian authorities have over the past few years arrested several Al Jazeera employees, raising concerns over media freedom in the country.

In May, a Cairo court sentenced a former editor-in-chief of Al Jazeera Arabic, Ibrahim Helal, to death, charging him in absentia with endangering national security.

Al Jazeera's Baher Mohamed, Mohamed Fahmy and Peter Greste - along with seven colleagues outside the country - were accused of spreading "false news" during their coverage of the aftermath of the military overthrow of then-president Mohamed Morsi in 2013, the year they were taken into custody.

Mohamed and Fahmy spent 437 days in jail before being released. Greste spent more than a year in prison.