Al-Maliki, under pressure from his Washington backers to rein in sectarian violence that is pushing Iraq to the brink of civil war, also said Iraqi politicians would meet in mid-December to try to reconcile rival communities.
The ambushed employees were from the Shia Endowment, a foundation that oversees religious sites and mosques.
Salah Abdul Razzaq, a spokesman for the organisation, said: "It's clear that this crime is aimed at stoking sectarian strife among Iraqis. The terrorists are trying to portray these crimes as a sectarian conflict."
Abdul Razzaq said 14 employees were killed and eight wounded when armed men forced the bus to stop on a motorway in the Qahira district.
Car bomb
Interior ministry sources said armed men first set off a car bomb and then sprayed the bus with bullets.
The sources put the toll at 15 dead and seven wounded.
In a separate attack near a petrol station in a religiously mixed area in southern Baghdad, three car bombs killed 16 people and wounded 25.
The violence came on the eve of the release of a report by the bipartisan Iraq Study Group co-chaired by the former secretary of state, James Baker, which is to offer George Bush, the US president, proposals on stabilising Iraq and reducing the US presence.
The bipartisan group's recommendations could include US troops taking a back-seat to Iraqi forces and seeking help from neighbouring Iran and Syria.
Growing influence
Al-Maliki, a Shia who has failed to ease violence since he took office seven months ago, did not name which neighbouring countries he was sending envoys to.
US and Iraqi officials have long accused Syria of doing too little to stem the flow of foreign fighters and weapons across its long, porous border.
Last month, Iraq and Syria agreed to restore full diplomatic relations after a break of nearly a quarter of a century.
Washington and many of Iraq's Sunni-ruled neighbouring states such as Jordan and Saudi Arabia are suspicious of non-Arab, Shia Iran's growing influence in Iraq, where Shia came to power after Saddam Hussein, the former president and a Sunni, was overthrown.
"We are aiming to have this conference in Iraq because it will be a sign of support to Iraq, but it is not decided yet. We have formed the delegations which will be sent," al-Maliki said.
"After talking with the governments, we will decide whether the conference will be for neighbouring countries, or beyond, a regional or international conference."