Obama White House
Russian hackers reportedly gained access to Obama's personal White House schedule. Getty

Russian hackers who breached White House cyber security and stole President Barack Obama's private schedule gained access to more sensitive information than has previously been admitted, according to reports.

The seriousness of the security breach was underplayed when the hack was first reported last October, with White House officials claiming that only non-sensitive networks were affected. However CNN now claims that the hackers were able to access information on Obama's schedule.

Though not classified, such information is highly prized by foreign intelligence agencies.

The attack was routed through a labyrinth of computers, but codes and markers link the attack to the Russian government, experts believe.

Hackers sent a phishing email through a hijacked State Department account, which was opened, allowing them to infiltrate the system.

Having accessed the State Department, they were able to infiltrate the White House computer system, according to the report.

The White House confirmed that there was "an event" last year.

"We have classified systems that are secure. We don't talk about where cyber intrusions originate from," said deputy national security adviser Ben Rhodes.

National Security Council spokesperson Mark Stroh said the report was "speculating" on who was behind the attack.

"Any such activity is something we take very seriously," Stroh said.

"We took immediate measures to evaluate and mitigate the activity."