ISLAMABAD, Pakistan (CNN) -- Al Qaeda or one of its affiliates was probably behind an explosion outside the Danish embassy in Pakistan, Denmark's intelligence service said Tuesday.
Authorities said a suicide car bomber pulled up next to the embassy and detonated explosives.
The massive blast in the capital city of Islamabad killed at least six people, injured 24 others and damaged the embassy wall and a neighboring United Nations building. It also left a 4-foot-deep crater in the road.
"It is ... the assessment that al Qaeda or an al Qaeda-related group is the possible perpetrator behind the attack," said Carsten Thomsen, spokesman for the Danish Security and Intelligence Service.
No group has claimed responsibility for the attack.
Danish embassies in predominantly Muslim countries, such as Pakistan, have been the scene of protests since Danish newspapers reprinted cartoons that Muslims say insult their prophet.
The Islamabad attack came six weeks after al Qaeda deputy leader Ayman al-Zawahiri, in a video message, called on Muslims to retaliate against Denmark for the caricatures.
"The assessment as of now -- which is not a final assessment -- is that it seems to be a terror-related attack using a car bomb," Thomsen said. "And that it is, all in all, likely that the Danish embassy has been the target of the attack, regardless of the fact that the embassy lies in close proximity to other foreign representations. "
Thomsen said the Danish Security and Intelligence Service did not discount the possibility that the attack may have been the work of some other militant groups.
"We should note that some other militant Islamic groups in Pakistan also might have the intention and capacity to target Danish targets in Pakistan," he said.
Authorities said a suicide car bomber pulled up next to the embassy about 1 p.m. and detonated explosives 10 to 15 feet from the embassy gate.
The blast was heard more than two miles away.
Among the dead was a security guard working at a nearby United Nations Development Programme building, said U.N. Secretary General Ban Ki-moon in a statement. The dead also included a gardener and a custodian who worked in the Danish embassy, the country's foreign ministry said.
None of the dead was a foreigner, although a Brazilian citizen of Pakistani descent was among the wounded, Pakistani police said.
The blast sheared off the embassy's front wall, blew out window panes and kicked in its metal front gate. It destroyed at least 20 cars, reducing many to twisted hunks of metal.
After the attack, the Pakistani government said it was beefing up security at embassies and foreign missions throughout the city. And Prime Minister Yousaf Raza Gilani told his Danish counterpart that his administration will step up its fight against terrorism.
The bombing, he said, will "redouble our resolve not only to apprehend the culprits of this heinous act but also to continue on our avowed path to fight terrorism and extremism," according to state media.
The country experienced a month-long lull in attacks after a new government took office in March and set on a course to negotiate with militants. But attacks have picked up again. Authorities have blamed Islamic militants for carrying them out.
The explosion was the first deadly attack in Islamabad since a bomb was hurled over a wall surrounding an Italian restaurant March 15. That explosion killed a Turkish woman and wounded 12 people, including four U.S. FBI agents.
The renewed controversy over the cartoons of Islam's prophet Mohammed flared in February when several newspapers in Denmark reprinted them. The papers did so after Danish authorities arrested several people who were said to be plotting a "terror-related assassination" of the cartoonist, Kurt Westergaard.
Westergaard's cartoon depicted the prophet wearing a bomb as a turban with a lit fuse. He said he wanted his drawing to say that some people exploited the prophet to legitimize terror. However, many in the Muslim world interpreted the drawing as depicting their prophet as a terrorist.
Islam generally forbids any depiction of the prophet, even a favorable one, fearing that it may lead to idolatry.
Two years ago, demonstrations erupted across the world after some newspapers printed the same cartoons. Some protests turned deadly.
The protests prompted Danish officials to temporarily close the embassy in Islamabad.