The State Department faces thousands of cyber attacks every day, a top US official said Friday, refusing to confirm that hackers who breached their system in November were reportedly still lurking in the network.

"We have robust security to protect our computer systems and our information, which includes access to an unclassified, open-net system," State Department spokeswoman Jen Psaki said.

But she acknowledged the attacks on an increasing list of public and private institutions were growing more sophisticated.

"We deal successfully with thousands of attacks every day, and we deal with them in conjunction with other relevant government agencies," Psaki said.

The Wall Street Journal reported that three months after the November breach forced the State Department to shut down its unclassified email network, the government had not been able to evict the hackers from the system.

Sources told the business daily that "each time investigators find a hacker tool and block it, the intruders tweak it slightly to attempt to sneak past defenses."

The original attack appeared to have been linked to the Russian government, and the hackers had taken State emails related to the crisis in Ukraine, the sources told the Journal.

Psaki refused to confirm or reveal any details, adding only "we work every day to fight back on these attacks and take a number of steps."

The Journal's report comes a week afer President Barack Obama led a cybersecurity summit at Stanford University and called on Silicon Valley to put aside distrust of the government to become allies in defending cyberspace from terrorists, hackers and spies.

He also signed an executive order aimed at spurring better and faster sharing of cyber threats between the private sector and government.

"This has to be a shared mission," Obama said.