The number of journalists killed worldwide in retaliation for their work nearly doubled this year, according to a report by the Committee to Protect Journalists.

The New York-based organisation found that 34 journalists died in retaliation killings in the year to 14 December, while at least 53 were killed overall. That compares with 18 retaliation killings among 47 deaths documented by the committee in 2017.

The annual report, issued on Wednesday, includes the killing of the Washington Post columnist Jamal Khashoggi, a native of Saudi Arabia fiercely critical of its regime. His death on 2 October inside the Saudi consulate in Istanbul has led to tremors on the global political scene around allegations that the Saudi crown prince, Mohammed bin Salman, was involved.

Khashoggi lived in self-imposed exile in the US, and had gone to the Saudi consulate to formalise his divorce, where he was strangled and dismembered – allegedly by Saudi agents.

Asked whether he believed the crown prince had ordered Khashoggi’s murder, Donald Trump said last month: “Maybe he did and maybe he didn’t.” While the US president condemned the violence against journalists, the committee noted he had called them “enemies of the people”.

In addition to retaliation killings, journalists have died in combat or crossfire, or on other dangerous assignments. The deadliest country for journalists this year was Afghanistan, where 13 journalists were killed, some in attacks by suicide bombers and claimed by the militant group Islamic State, according to the report.

The media freedom group Reporters Without Borders said on Tuesday the US made it into the top five deadliest countries for journalists this year for the first time, with six journalists dying including four who were among the five people killed when a gunman opened fire in the offices of the Maryland newspaper Capital Gazette on 28 June.

The shooting, in which a sales associate was also killed, was the deadliest single attack on the media in recent US history. The gunman had threatened the newspaper after losing a defamation lawsuit. Another two journalists died while covering extreme weather.

In addition, the committee said an increasing number of journalists were being imprisoned.

“The context for the crisis is varied and complex, and closely tied to changes in technology that have allowed more people to practise journalism even as it has made journalists expendable to the political and criminal groups who once needed the news media to spread their message,” the committee said in its report.

Time magazine last week recognised jailed and killed journalists as its “person of the year”, including Khashoggi, Maria Ressa, who was imprisoned in the Philippines, Wa Lone and Kyaw Soe Oo who were imprisoned in Myanmar, and staff at the Capital Gazette.

Journalists have also died this year in Slovakia, where the 27-year-old investigative reporter Ján Kuciak was shot while looking into alleged corruption. Last year in Malta, Daphne Caruana Galizia, on a similar mission, was killed by a bomb placed in her car. At least four journalists were murdered in Mexico and two in Brazil, and two Palestinian journalists were shot and killed by Israeli soldiers during protests in the Gaza Strip, according to the report.

In Syria and Yemen, two countries gripped by civil war, the fewest journalists were killed since 2011. Three died in Yemen, while in Syria the committee recorded nine deaths compared with a high of 31 in 2012. The drop may be due to limited access or extreme risks that discourage media visits, the committee said.