BAGHDAD - US Defense Secretary Ash Carter arrived in the Iraqi capital of Baghdad on Thursday on an unannounced visit to meet with Iraqi leaders over the battles against the Islamic State (IS) militant group, an official television reported.

Carter is due to discuss the campaign of the US-led coalition against the extremist IS group with the Iraqi leaders, including Prime Minister Haider al-Abadi, the state-run Iraqiya channel said.

Carter's visit is his first to Iraq as Pentagon chief since he took office in February. He said he aims to have his own assessment of the situation on the ground.

His visit came as the Iraqi authorities announced on July 13 the start of a major offensive against IS militants to free key cities and towns in Iraq's largest province of Anbar province from IS militants.

Earlier this month, Iraq received four F-16 fighter jets from the United States, the first batch of a total of 36 such warplanes ordered by Baghdad to boost capabilities of the Iraqi security forces in their fight against the IS militants.

The security situation in Iraq has drastically deteriorated since June 2014, when bloody clashes broke out between Iraqi security forces and the IS militants.

The IS militants took control of the country's northern city of Mosul and later seized swathes of territories in Nineveh and other predominantly Sunni provinces.

In response to IS expansion, the United States sought to build a coalition of both world and regional countries to fight back the IS extremist group.

So far, the US-led coalition has been carrying out air strikes against the armed Islamist group in both Iraq and Syria.