WASHINGTON: The US official in charge of overseeing the billions of dollars being spent to rebuild Afghanistan announced his resignation Monday, just a week after he fired two of his top deputies in a major shake-up of the organization.

Arnold Fields’ decision to step down comes after key members of Congress urged President Barack Obama to dismiss him for incompetence and mismanagement. His critics claimed he failed to aggressively oversee the more than $56 billion the US has poured into Afghanistan since 2002 to rebuild schools, roads and other facilities.

Fields was appointed by President George W. Bush as the special inspector general for Afghanistan reconstruction in June 2008 after the job was created by Congress. His resignation is effective Feb. 4.

At the time Fields removed his top assistants for audits and investigations, he told The Associated Press the office needed new blood, and he said he did not dismiss his senior staff in order to save his own job. Fields, a retired Marine Corps major general, described the moves as the mark of a leader making tough decisions. He gave no indication he planned to resign.

In a brief statement, Fields said he would use the next month to ensure a smooth transition of his duties to his replacement, who has not yet been named. The statement did not say why Fields decided to resign. Susan Phalen, Fields’ spokeswoman, said only that he met with White House officials on Friday and told them of his plans to leave the post.

In a separate statement, the White House praised Fields for his ‘‘hard work and steadfast determination.’’

Fields had a rocky tenure as the watchdog charged with combating corruption in the vast Afghanistan reconstruction effort. In December 2009, three senators complained to President Barack Obama that Fields had failed to hire enough staff and issued too few audits and investigative reports at a time when aggressive, independent oversight of how US tax dollars were being spent in Afghanistan was more important than ever.

Last summer, reviews of Fields’ office by the Council of the Inspectors General on Integrity and Efficiency found multiple problems, including a failure to meet minimum standards for conducting investigations. That led to calls from several senators for Obama to fire Fields. He didn’t.

In November, a Senate oversight subcommittee on government contracting grilled Fields at a hearing. Sen. Claire McCaskill, the subcommittee’s chair, said the organization — known as SIGAR — had completed only a few audits of Afghanistan reconstruction contracts even though thousands of contracts worth billions of dollars have been awarded for work in the war-torn country.

Fields was also criticized for giving Joseph Schmitz, a former Pentagon inspector general, a two-month consulting contract worth $95,000.

Fields sought to defend his performance, saying he built the oversight office from scratch to an organization of more than 120 people amid struggles to get adequate funding for the office.

He failed to sway his detractors.

‘‘I don’t think that you’re the right person for this job,’’ McCaskill told him at the hearing.

In a statement issued Monday, McCaskill and three other senators who had pressed for Obama to dismiss Fields applauded his decision to resign.

‘‘With billions of dollars being spent in Afghanistan, our country must have top notch leadership at the agency responsible for rooting out the waste and fraud that can jeopardize our efforts,’’ McCaskill said.

‘‘I hope that his departure will allow the agency to turn over a new leaf and finally begin to do the important contracting oversight work we so desperately need.’’