An unnamed administration official told US media that there was no indication any data had been removed.
The conservative Washington Free Beacon reported on Sunday that hackers linked to the Chinese government had breached the White House Military Office.
The White House would not say if the attack originated in China, describing it as a "spear-phishing" attempt.
"Spear-phishing" typically works by sending fake e-mails that look like legitimate correspondence and links to a malicious website or file attachment.
"These types of attacks are not infrequent and we have mitigation measures in place," the official, who was not authorised to speak on the record, told the Associated Press and other US media.
Cyber-attacks from Chinese-linked hackers have been an increasing concern among US government offices, including the Pentagon, the top US cyber defence official told Reuters last week.
"Their level of effort against the Department of Defense is constant," Rear Admiral Samuel Cox said.
In 2011, Google blamed computer hackers in China for a phishing effort against Gmail accounts of several hundred people, including senior US government officials and military personnel.
That November, senior US intelligence officials for the first time publicly accused China of systematically stealing American high-tech data for its own gain.
There were few details on the attack, which was reported by the Local Coordination Committees activist group and the Britain-based Syrian Observatory for Human Rights.
The Observatory said it was aware of up to 100 killed from sources on the ground, but the group had only confirmed the names of 30 people so far.
Death tolls are nearly impossible to independently verify in Syria, where the government restricts journalists and where more than a year of violence has convulsed much of the country.
There were few details of the violence in Hama's Tremseh area.
Activists say more than 17,000 people have been killed since the uprising against President Bashar Assad began in March 2011, and he is coming under growing international pressure to stop the violence. But as the bloodshed continues, and the conflict morphs into an armed insurgency, hopes for a peaceful transition are dimming.
The latest report of violence came in the wake of the highest-level defector yet from President Bashar Assad's regime – his ambassador to Iraq.
Defections from the Syrian regime have stirred hopes in the West Assad's inner circle will start abandoning him in greater numbers, hastening his downfall.
But the tightly protected regime has largely held together over the course of the 16-month-old uprising, driven by a mixture of fear and loyalty.
The latest official to flee, Ambassador Nawaf Fares, announced that he was joining the revolution, asserting Thursday that only force will drive Assad from power.
"There is no road map ever with Bashar Assad, because any plan, any statement that is agreed on internationally he delays on and ignores," Fares told the Al-Jazeera satellite channel. "There is no way that he can be pushed from power without force, and the Syrian people realize this."
Syria's Foreign Ministry denounced Fares, saying he should face "legal and disciplinary accountability."
In Washington, State Department spokesman Patrick Ventrell hailed what he called the "first major diplomatic defection," adding: "We think this a wider sign that the regime is feeling the pressure. The pressure is up and the regime is really starting to fall apart."
Fares is the second prominent Syrian to break with the regime in less than a week. Brig. Gen. Manaf Tlass, an Assad confidant and son of a former defense minister, defected last week, but has not spoken publicly.
French Foreign Minister Laurent Fabius said Tlass has been in contact with the Syrian opposition. He would not comment on reports that Tlass was in Paris.
"I know that there is some closeness between the opposition and the general... Contact has been made," Fabius told journalists in Paris.
Assad's regime has suffered a steady stream of low-level army defectors, who have joined a group of dissidents known as the Free Syrian Army, now numbering in the tens of thousands. There have been several high-level defections in the past – including a Syrian fighter pilot who flew his plane to neighboring Jordan during a training mission in June in a brazen move.
