The top US commander in Afghanistan, Gen John Allen, is being investigated for allegedly sending "inappropriate emails" to a woman at the centre of a scandal which ended the career of CIA head David Petraeus.
The emails were said to be to Jill Kelley, whose complaint led the FBI to uncover Gen Petraeus's extramarital affair with Paula Broadwell.
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Gen Allen denies any wrongdoing.
His nomination to head Nato in Europe is now on hold, the White House says.
Gen Allen was due to face a Senate confirmation hearing for the post of Supreme Allied Commander in Europe, but that has been suspended at the request of Defence Secretary Leon Panetta, a spokesman said.
David Petraeus, who preceded Gen Allen as head of US forces in Afghanistan, resigned on Friday as head of the Central Intelligence Agency (CIA) over his affair with Mrs Broadwell.
In a statement, Mr Panetta said that the Federal Bureau of Investigation (FBI) referred the Gen Allen issue to the Pentagon on Sunday. The defence secretary, currently in Australia, then ordered an investigation on Monday.
He has encouraged the US Senate to act quickly on approving Gen Allen's successor in Afghanistan, Gen Joseph Dunford.
Gen Allen was due to face a Senate confirmation hearing on Thursday. If approved, as had been widely expected, he was expected to take up his appointment heading the Nato-led International Security Assistance Forces (Isaf), in early 2013.
Mr Panetta praised the general's work in Afghanistan, saying his leadership had ensured "significant progress" in improving security for Afghans.
Gen Petraeus had been due to testify before a closed-door congressional committee meeting about the 11 September 2012 attack on the US consulate in Benghazi, Libya, that killed the ambassador and three other Americans.
Acting CIA director Michael Morell will testify in his place on Thursday, although Gen Petraeus may be called at a later date.
Mr Panetta has criticised the FBI for not informing members of US Congressional intelligence committees of its investigation.
FBI officials are scheduled to brief the Senate and House intelligence committees on Tuesday about the case.
The widening of the Petraeus affair increases the focus not only on Gen Allen but on the woman he is alleged to have exchanged emails with.
Investigators are reported to have found 20-30,000 pages of documents, many of them emails between Gen Allen and Jill Kelley, 37, a married woman from Tampa, Florida.
A senior defence official would not say whether they involved sexual matters or whether they are thought to include unauthorised disclosures of classified information, the Associated Press reports.
Mrs Kelley had already been named in the Petraeus scandal after she told the FBI early in the summer she had received anonymous harassing emails.
When the FBI investigated, it traced the emails to Paula Broadwell, bringing to light her affair with Gen Petraeus.
Late on Monday, the FBI carried out a search of Mrs Broadwell's home in Charlotte, North Carolina.
Mrs Kelley is married to a prominent cancer specialist in the Tampa area and works as a volunteer with wounded veterans and military families at the MacDill Air Force Base in the city.
The base is headquarters of the US Central Command, formerly headed by David Petraeus.
She is a family friend of him and his wife Holly, but no evidence has emerged that she ever had an affair with the CIA director.
Gen Allen, a four-star Marine general, succeeded Gen Petraeus as the top American commander in Afghanistan in July 2011.
He was deputy commander of Central Command before taking over in Afghanistan. He is also a veteran of the Iraq war.
Gen Petraeus's affair with Mrs Broadwell, who co-wrote a biography of the general, began following his retirement from the army and about two months after he became director of the CIA. He said it ended about four months ago.
Gen Petraeus joined the CIA in September 2011 after heading international forces in Iraq and later in Afghanistan.
The BBC's Bilal Sarwary, in Kabul, says the scandal is making headlines and has shocked many people in Afghanistan, where Gen Allen had built good relations with leaders and commanders.
Some Afghan officials are asking how he found time to write thousands of emails when he was in charge of running a war, our correspondent says.