At least 18 Honduran journalists, human rights and political activists have been killed in apparent post-coup political violence since the start of the year, a rights group said Tuesday.

In a report published Monday, Human Rights Watch called on authorities to investigate a total of 47 cases of threats or attacks on opponents of the 2009 coup since President Porfirio Lobo assumed power in January.

'A year and a half after the coup in Honduras, the consequences for human rights are still being felt,' said Jose Miguel Vivanco, Americas director at the New York-based group.

After the coup, security forces committed serious human rights violations -including excessive force against demonstrators and arbitrary detentions- as well as illegitimate restrictions on freedom of expression and assembly.

'It is clear from our findings that until Honduran authorities take concrete steps to reduce impunity and stop the attacks, it will be very difficult to restore trust in the country's democratic system', the group said.

According to Human Rights Watch, the killings appeared to have been politically motivated, citing as an example Julio Benitez, a coup opponent who received numerous threatening phone calls before he was shot dead in February.

'The lack of accountability -and ongoing violence and threats- have had a chilling effect on free speech and political participation in Honduras, particularly among those who opposed the 2009 coup,' the report said.

The Human Rights Unit of the Attorney General's office has filed 20 cases of alleged government violations, acquitting eight of the defendants with the remainder pending, according to the rights group.

'This lack of progress is primarily the result of the lack of cooperation with, and support for, the Human Rights Unit on the part of other state institutions,' including security forces, it said.

On June 28, 2009 soldiers forced then-president Manuel Zelaya from power and exiled him after the wealthy cattle rancher swerved to the left and became an ally of Venezuela's fiery leader Hugo Chavez.

The coup was condemned by the international community. The OAS (Organisation for American States) suspended Honduras's membership, and many Latin American governments withdrew their ambassadors from the country.

Many in the region, including powerhouse Brazil, refuse to recognize Lobo, who won power in polls held under an unelected interim regime last November and took office in January.

But the United States, the traditional Honduras backer, and the European Union restored economic and military cooperation after condemning the coup in one of the region's poorest countries.

Last November, the Chief Prosecutor at the International Criminal Court (ICC), Luis Moreno-Ocampo has announced that his Office has opened preliminary investigations in Honduras.

The ICC's investigation will specifically focus on the June 2009 coup d'état in which Manuel Zelaya was removed from power in the Central American state.