A doctor from Pakistan who said he wanted to “fight on the frontline” for the Islamic State group was sentenced on Friday to 18 years in a U.S. prison, federal prosecutors said.

The doctor, Muhammad Masood, 31, of Rochester, Minn., pleaded guilty last year in U.S. District Court in St. Paul to attempting to provide material support to a terrorist organization.

Prosecutors said that Mr. Masood had expressed a desire to government informants to conduct “lone wolf attacks” in the United States or fight and work as a combat medic for ISIS in the Middle East.

Mr. Masood’s prison term will be followed by five years of supervised release under the sentence handed down by Judge Paul A. Magnuson of U.S. District Court.

Mr. Masood, who worked at a research clinic in Rochester, was arrested in March 2020 at Minneapolis-St. Paul Airport before he could board a flight to Los Angeles, where he planned to board a cargo ship to travel to the Middle East and join the terrorist organization, prosecutors said.

In a phone interview on Friday, Mr. Masood’s lawyer, Jordan Kushner, called the sentence “extremely harsh” given his client’s history of mental illness.

In court, Mr. Kushner alluded to a report from a psychiatrist who concluded that Mr. Masood’s actions should be understood “not as an act of devotion to violent extremism and the aims of ISIS, but as a consequence of his mental illness and multiple stressors.”

But prosecutors had asked the judge to impose the maximum penalty of 20 years in prison, saying in court papers that Mr. Masood “chose a path to become a soldier and combat medic for a terrorist organization” despite having a successful career.

The investigation into Mr. Masood began in January 2020 when the authorities learned that he had sought help on an encrypted social media platform on “making hijra,” prosecutors said. The term is commonly used in Islamic State circles to indicate travel to territories controlled by ISIS for the purpose of “violent jihad,” according to an affidavit prepared by the Federal Bureau of Investigation.

Mr. Masood had been living in the United States on an H-1B visa since 2018, and held a medical license in his native Pakistan.

On the encrypted social media platform, he connected with two government informants who he believed to be Islamic State members who would help him join the organization, prosecutors said.

In conversations with the informants, Mr. Masood expressed being “sick” of living in the United States, and mused about carrying out lone wolf terrorist attacks, according to the affidavit.

Ultimately, Mr. Masood said he belonged on the frontline, where he could be “a combat medic … and also fight,” according to the affidavit. He initially devised a plan to reach Syria by flying from Chicago to Jordan in March 2020, but that plan was upended when the coronavirus pandemic prompted Jordan to close its borders, prosecutors said.

Instead, Mr. Masood purchased a ticket for a flight from Minnesota to Los Angeles, the authorities said. On March 19, 2020, he was arrested by federal agents at Minneapolis-St. Paul Airport.