The State Department has also hired outside law enforcement experts to see how the department handles such allegations, a department spokeswoman said.
Allegations in an internal draft department memo first reported by CBS News that senior State Department officials "manipulated, influenced or simply called off" investigations of eight cases of alleged misconduct, including sexual assaults and engaging with prostitutes, "are very serious," Senate Foreign Relations Committee Chairman Robert Menendez, D-N.J., said through a spokesman.
"The committee is looking into the alleged actions, and this situation underscores the need for an appointment of a permanent inspector general for the Department of State to assure that allegations of this sort are quickly, fully and appropriately addressed," Adam Sharon said in an email cited by Politico.
House Foreign Affairs Committee Chairman Ed Royce, R-Calif., told MSNBC allegations of a State Department coverup of serious law violations were "very concerning."
"It's not just what happened at the lower level," Royce said of the allegations of impropriety.
"You have a situation where senior political appointees at the State Department leaned in and, in these reports, clearly attempted to call off investigations, manipulated investigations, not just on misconduct but on allegedly criminal activity," he told the network.
The State Department steadfastly denied senior officials ignored illegal behavior by diplomats, calling it "preposterous."
"We hold all employees to the highest standards. We take allegations of misconduct seriously and we investigate thoroughly," spokeswoman Jen Psaki told reporters Monday.
"All cases mentioned in the CBS report were thoroughly investigated or [are] under investigation, and the department continues to take action," she said.
The department's internal watchdog, the Office of the Inspector General, "has taken the further step of requesting an additional review by outside experienced law enforcement officers ... so that officers with law enforcement experience can make expert assessments" about whether the State Department's law enforcement arm, the Bureau of Diplomatic Security, has the independence necessary to do its jobs effectively, Psaki said.
The outside review was ordered "long before" CBS reported the allegations, she said.
"I'm not going to talk about specific cases, but I can say broadly that the notion that we would not vigorously pursue criminal misconduct in any case is preposterous," Psaki said. "And we've put individuals behind bars for criminal behavior. There is record of that. Ambassadors would be no exception."
Allegations cited in the Oct. 23, 2012, memo reported by CBS include claim an ambassador still serving in Europe routinely "ditched his protective security detail in order to solicit sexual favors" from prostitutes in a park.
The unnamed ambassador issued a statement denying any impropriety.
"I live on a beautiful park ... that you walk through to get to many locations, and at no point have I ever engaged in any improper activity," the statement said.
Other allegations include a report a State Department security official in Beirut "engaged in sexual assaults" on foreign nationals hired as U.S. Embassy guards, diplomatic security officers for Hillary Clinton, when she was secretary of state, having an "endemic" issue with hiring "prostitutes while on official trips in foreign countries" and an "underground drug ring" operating near the U.S. Embassy in Baghdad providing drugs to State Department security contractors.
Undersecretary of State Patrick F. Kennedy, a career foreign service officer, denied he impeded any investigation.
"It is my responsibility to make sure the department and all of our employees -- no matter their rank -- are held to the highest standard, and I have never once interfered, nor would I condone interfering, in any investigation," he said in a statement.
A senior State Department official told ABC News the European ambassador was scolded for bad judgment -- for walking in a park known for sexual trysts -- but the official insisted the investigation found no evidence of wrongdoing.
The official said in that case and others, full investigations were done and in many cases insufficient evidence was found to warrant prosecutions or in-house discipline.