Iraq’s president has appointed Haidar al-Abadi as the new prime minister ending the eight-year rule of Nouri al-Maliki as the country faces its biggest post-invasion security crisis.

Washington, which worked closely with Al-Maliki after his appointment in 2008, has congratulated Al-Abadi on his appointment and asked Al-Maliki to step down.

However, Al-Maliki said that his replacement was a “violation of the constitution” and has refused to step aside, rejecting the nomination in two television appearances.

Al-Abadi, if he is able to form a government, faces the hard task of dealing with Iraq’s crippling security crisis with Islamic State in Iraq (Isis) militants controlling vast swathes of the north and west of the country.

As the leader of the Shia-based Dawa party, Al-Maliki was criticised during his two terms for marginalising the country’s large Sunni minority, which contributed to the growing support for Isis in areas with large Sunni populations.

Al-Maliki also lost the support of the Kurdistan Regional Government (KRG) after a dispute over national budget payments and the Kurdish enclave’s right to export oil.

Al-Abadi is a member of Al-Maliki’s State of Law coalition, which gained the largest share of the vote in the parliamentary elections. In July, he was appointed deputy speaker of parliament.