Map: Tikrit, Iraq

Baghdad (CNN) -- Iraqi authorities executed four al-Qaeda senior leaders after they were convicted of terrorism, the nation's Ministry of Justice reported Monday.

Manaf Abdul-Raheem Abdul-Hameed al-Rawi, the leader of the so-called Islamic State of Iraq in Baghdad, was among the four who were executed by hanging Monday morning.

The executions came on a day when a suicide bomber drove an oil tanker into a police station in central Tikrit, killing at least nine people and wounding 20, an official with Iraq's interior ministry said.

Tikrit is about 160 kilometers (100 miles) north of Baghdad. It's a predominantly Sunni town located in Salaheddin province and is the hometown of former Iraqi leader Saddam Hussein.

In Baghdad, two policemen and a civilian were shot dead by gunmen in a pair of incidents, the Interior Ministry said.

Two other people were wounded, according to the ministry official.

The security situation in Iraq was supposed to be the subject of a parliamentary session Monday attended by Iraqi Prime Minister Nuri al-Maliki, who is also the military's commander-in-chief, along with the acting minister of defense, the acting minister of interior, the commander of Baghdad operation command and the head of Iraqi intelligence services.

But al-Maliki did not attend, instead sending a letter to the speaker of the parliament, Ossama al-Nujaifi, saying that to discuss such sensitive issues in an open session could harm national security.

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The prime minister asked for a closed session including only the leaders of the Iraqi political blocs and the members of the parliamentary commission for security. He asked that the meeting be at his office to guarantee confidentiality.

After the letter was read to Iraqi lawmakers by al-Nujaifi, they agreed to schedule a closed session next week, but insisted that al-Maliki should come to the parliament and discuss security developments in front of all Iraqi lawmakers.

"We will not give up our constitutional duty. It can be in a closed session but al-Maliki should attend the parliament and speak in front of all members of parliament," al-Nujaifi said.

A few members shouted that al-Maliki should adhere to the Iraqi constitution and not give weak excuses for not attending Monday's session.

The session was broadcast live on a number of Iraqi satellite television carriers. However, Al Iraqiya state television aired only part of the session.

Executions such as those conducted by the government Monday morning were addressed in Amnesty International's annual report, issued in March. The death penalty was suspended after the 2003 U.S.-led invasion but was quickly restored by the first post-invasion Iraqi government. Executions resumed in 2005.

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Since then, at least 447 prisoners have been executed, including Saddam Hussein, some of his main associates, and alleged members of armed groups. Hundreds of prisoners await execution on death row.

Iraq, where 129 prisoners were hanged in 2012, is now one of the world's leading executioners, Amnesty International said.

With Monday's violence, April's start was similar to March's end. On Sunday, at least seven people were killed and more than 17 were wounded in violence across the country, police officials said.

In figures compiled by Iraq's Interior, Defense and Health ministries for the month of March, 163 Iraqi civilians, policemen and soldiers were killed in acts of violence across the country and 256 were wounded. The figures don't include the Kurdistan region in northern Iraq.

Since the pullout of the U.S.-led coalition forces, Iraqi security forces have sometimes struggled to maintain order. Daily violence has drastically dropped across the country since 2008, but attacks continue.