A national newspaper, The Asahi Shimbun, said on Thursday the continued bloodshed in Iraq had frightened Japanese officials so badly they were unlikely to send troops this year.
The fears had been fanned by Tuesday's suicide bombing of the UN headquarters in Iraq, which left at least 23 people dead.
The Asahi quoted government sources as saying if Japanese troops were sent to Iraq at all it would be next year at the earliest.
And Defence Minister, Shigeru Ishiba, confirmed on Wednesday that dispatching troops this year could be difficult.
"Sending them within the year may be possible, but it's a fact that they are unlikely to be dispatched soon.
"The fact that the UN, which engages in humanitarian aid, was attacked shows that our troops may not be safe just because they are solely engaged in humanitarian activities," Ishiba said.
Tokyo's decision to send troops for a humanitarian mission has already prompted worries in Japan for their safety.
Officials said a final decision on the role of Japanese troops would be made after a fact-finding mission surveyed the situation in Iraq.
Fischer (L) angered the Americans
by opposing the Iraq war
The survey mission was expected to go this month, but a defence ministry spokesman said nothing had yet been decided.
Prime Minister Junichiro Koizumi has been determined to send troops to Iraq despite a poll that showed more than half of Japanese voters oppose the move.
Japanese soldiers have not fired a shot in combat since Japan's World War Two defeat, in line with its pacifist constitution.
Meanwhile, Germany has reiterated its refusal to despatch soldiers to Iraq to provide extra security and stabilise the country.
"The question of a military involvement does not arise," Foreign Minister, Joschka Fischer, told the Sueddeutsche Zeitung on Thursday.
His remarks came amid criticism from German opposition leaders that while Berlin is calling for greater security in Iraq, it is doing nothing to provide it.
"We would have wished for a central role of the UN, but some of our partners were of another opinion. We have to accept that," Fischer said.
"We did not take part in the war (against Iraq) for good reasons... But that does not mean we are in anyone's debt. We don't feel under any pressure," he added.