Bart Jansen

USA TODAY

Afghanistan plans for a military offensive in coordination with U.S. troops against the Islamic State have become more urgent as the country marked a national day of mourning Sunday for 80 people killed and 231 injured in Kabul's worst attack in 15 years.

Afghan President Ashraf Ghani recently announced a major assault in the Nangarhar province along the country’s eastern border with Pakistan.

The effort, nicknamed Shafaq, or “dawn” in Pashto, would be the largest since most international combat troops withdrew in 2014. President Obama issued a directive in June allowing the U.S. military to work with Afghan forces on non-combat missions.

About 9,800 U.S. troops are in Afghanistan. Obama pledged this month to keep at least 8,400 troops in the country through 2017, which delayed plans to drop the number to 5,500 by the end of this year.

Planning for the military offensive later this month became more urgent as the Islamic State’s online news agency Aamaq claimed responsibility for Saturday's suicide bombing, the first Islamic State attack in the Afghan capital, the Associated Press reported.

"I promise you, I will take revenge against the culprits,” Ghani said in a televised address after the attack.

The blast came during a march by thousands of ethnic Hazaras who demanded the rerouting of a power line through the impoverished province of Bamiyan. Most Hazaras are Shiite Muslims, while most Afghans are Sunni. It was the first Islamic State attack on Kabul and the worst since a Taliban insurgency began 15 years ago.

Saturday's attack raised concerns about sectarianism, and the Interior Ministry announced a ban on public gatherings and demonstrations to avoid any inter-communal strife. The ban on public gatherings would not apply to the funerals for the bombing victims.

80 dead in Islamic State suicide bombing in Kabul

Ghani attended a memorial prayer service Sunday in a mosque on the grounds of the presidential palace, his spokesman Haroon Chakhansuri said.

The Islamic State has had a presence in Afghanistan for the past year, mainly in Nangarhar. The eastern province is strategic because it produces agricultural goods and provides a thoroughfare for exports to Pakistan and elsewhere.

"It is like a second capital," Afghan Army Gen. Shir Mohammad Karimi said, according to the AP. "If Nangarhar falls, Kabul will become a battleground every day.”