At least 18 people were killed and some 80 others injured when a suicide minibus bomber struck a police building in the central Baghdad Tuesday, in the wake of three deadly blasts targeting the city's hotels Monday which left at least 36 dead and dozens wounded.

The series of car bomb attacks have raised fresh concerns about worsening security situation in Iraq, as the war-torn country is preparing for its March parliamentary elections.

Monday's suicide car bombs, which struck several hotels in the Karrada district, are the most serious since the Dec. 8 series of similar bombings that left at least 127 dead in the capital, echoing a warning by both U.S. troops and Iraqi officials that violence will increase ahead of the election.

The explosions have occurred as different political blocs have been trying to resolve a recent dispute involving some 511 candidates being barred from running in the election for their suspected links to the former president Saddam Hussein's outlawed Baath Party.

Although 54 candidates have been erased from the blacklist after they submitted documents proving they had no links to the Baath Party, the fate of the rest remains unknown.

"There is a link between the violence, political tension and the election," said Sabah Al- Sheikh, a professor of politics at Baghdad University. "We notice violence increases when the election becomes close."

Militant groups tried to?use the political tension and committed bloody attacks in order to influence the political process, he said.

Sporadic small attacks have been frequent in parts of the country in the past few months with big ones occurring from time to time. On Jan. 14, three bombs -- a car bomb and two roadside bombs -- exploded near a major mosque in the southern holy city of Najaf, killing 25 and wounding dozens. Also on Jan. 12, Baghdad security forces said they foiled several suicide car bomb attempts in a large-scale search operation.

The perpetrators of these criminal attacks also want to send, by their actions, a message to everyone that they are still present in Baghdad, and that they can carry out large-scale attacks, according to the professor.

The latest bombings were similar in style to the massive attacks on Aug. 19, Oct. 25 and Dec. 8, which targeted government buildings. The Iraqi government has blamed the Al-Qaida-linked insurgents and loyalists of the outlawed Baath party for these attacks.

Monday's attacks targeted heavily protected hotels, which houses many foreigners and staff of western media organizations who work in Iraq.

By targeting major hotels, the armed groups aimed to make a big media hype, because such bombings will be publicized by such networks, especially as some of their offices have been damaged, the political analyst said.

Another factor which affects the security protection is the effectiveness of tens of hundreds of bomb detectors, which are widely used by Iraqi security forces on streets to detect car bombs.

Iraqi Prime Minister Nuri al-Maliki has ordered an immediate investigation into the use of explosives detection devices imported from a British company, after British media reported the devices could not detect bombs effectively.