Burma, which repressed protests in 2007, is ranked 170th
Security worries can erode freedoms even in democratic nations and undermine press freedom, media watchdog Reporters Without Borders (RSF) says.
The warning comes as the group publishes its annual 173-nation index of press freedom around the world.
RSF cited poor rankings by the US and Israel, and called for US political leaders to improve its situation.
Iceland topped the list while the US was 36th. North Korea, Turkmenistan and Eritrea were the lowest-ranked nations.
The Paris-based press freedom body described those three countries as "unchanging hells" in which the population is cut off from the world and is subjected to "propaganda worthy of a bygone age".
Yet it is peace rather than economic prosperity is the greatest guarantee of press freedom, RSF said.
While parliamentary democracies not engaged in any war did well, others - including the US and Israel - fare less well, the report said.
PRESS FREEDOM LIST 2008
172: North Korea
Although the US rose from its 2007 position of 48th to be ranked 36th, Israel fell two positions to 46th in the list.
The group has written to US presidential candidates Barack Obama and John McCain highlighting the need for more protection for reporters' confidential sources.
It also urged the two candidates to push for better working conditions for reporters in areas under US control in Iraq and Afghanistan.
"We sincerely hope that we can look to your administration for change from the policies of secrecy, ambiguity and avoidance that have hampered the work of a free press," RSF wrote to the presidential hopefuls.
The US has slipped from 111th to 119th in the rankings of press freedom enjoyed outside its territory, a position blamed on the ongoing conflicts in Iraq and Afghanistan.
Georgia ranks noticeably lower than in 2007, tumbling 54 places to 120th in the aftermath of its war with Russia during the summer.
But peaceful, democratic countries dominate the top 20 of the global index were European states, in particular Nordic countries such as Iceland, Norway and Finland.
Some of the newer democracies, including Estonia, Latvia and Slovakia, featured in the top 10. The UK was 23rd.
Some African countries fared better than in previous years, among them Namibia, Ghana and Mali. They rated better than Spain and Italy, which the report says have slipped because of political and Mafia interference.
China still languishes in the bottom 10 - despite hosting the 2008 Olympics - just worse than Saudi Arabia, Uzbekistan, Zimbabwe and Iran, although not quite as bad as Burma.
The data was compiled for the year ending 1 September 2008.