The purported Syrian meeting, possibly attended by Abu Musab al-Zarqawi himself, has led to one of the bloodiest periods since the US-led invasion two years ago.
Nearly 500 people have died - including an Iraqi general killed in a drive-by shooting on Wednesday - since the country's new Shia-dominated government was announced on 28 April.
Several Shia and Sunni Muslim clerics were among the victims, raising fears that sectarian tensions could ignite a civil war.
A internet audiotape purportedly by al-Zarqawi denounced Iraq's Shia as US collaborators and said killing them is justified.
"God ordered us to attack the infidels by all means ... even if armed infidels and unintended victims - women and children - are killed together," said the speaker purported to be al-Zarqawi.
"The priority is for jihad so anything that slows down jihad should be overcome." The tape could not immediately be authenticated.
A surge in violence has killed
about 500 people in Iraq
The Jordanian born al-Zarqawi and his key leadership have met at least five times in foreign countries during the conflict, most recently during the past 30 days in Syria, according to the senior US military official, who briefed reporters on condition he not be named.
He did not identify the other countries but said neighbouring Iran was not one of them.
He said the military obtained information during questioning of prisoners, from Iraqi military sources and field intelligence in determining that the most recent meeting had taken place in Syria.
Campaign against Syria
The Syrian Foreign and Information Ministries were unavailable for comment on the alleged terrorist gathering on their soil.
Iraq's presidential adviser for security affairs, General Wafiq al-Samarai, said he had "no information" about an al-Zarqawi meeting in Syria.
There have been 21 car bomb
attacks in May
Syrian political analyst Imad Fawzi al-Shueibi dismissed the report as "part of an organised campaign against Syria".
"Syria has no interest in (helping) al-Zarqawi," al-Shueibi said. "If al-Zarqawi and his group win in Iraq, they will turn the region into a fundamentalist nightmare."
The US, at the highest leadership levels, repeatedly has demanded that Syria do more to stop foreign fighters from entering Iraq across its porous 380-mile-long border.
At least one report suggested al-Zarqawi himself attended the talks, the US official said.
"He (al-Zarqawi) allegedly was not happy with how the insurgency was going; the government was getting stronger and coalition forces not being defeated," the official said.
"Some intelligence reports from captives showed that al-Zarqawi directed people to start using more vehicle-borne devices and (to) use them in everyday operations."
There have been 21 car bombings, mostly attacks, in Baghdad during May, compared with 25 such attacks in all of 2004, the official said. Nearly 130 car bombs have exploded or been defused since late February, he said.