WASHINGTON: Emails between US diplomats in Islamabad and State Department officials in Washington about whether to challenge specific US drone strikes in Pakistan are at the centre of a criminal probe involving Hillary Clinton's handling of classified information, the Wall Street Journal reported on Thursday.

The 2011 and 2012 emails were sent via the "low side", government slang for a computer system for unclassified matters as part of a secret arrangement to voice whether CIA drone strike went ahead, according to congressional and law enforcement, the Journal said.

Read: Email controversy hurts Clinton campaign for White House

Some of the emails were then forwarded by Clinton's aides to her personal email account, which routed them to a server she kept at her home in New York when she was secretary of state, the officials said.

Investigators have raised concerns that Clinton's personal server was less secure than State Department systems, and a recent report by the State Department inspector general found that Clinton had broken government rules by using a private email server without approval, undermining Clinton's earlier defences of her emails.

The still secret emails are a key part of the FBI investigation that has long dogged Clinton's presidential campaign, the officials told the Journal.

Clinton this week clinched the Democratic presidential nomination for the Nov 8 election.

The vaguely worded messages did not mention the "CIA," "drones" or details about the militant targets, officials said, according to the Journal.

The emails were written within the often narrow time frame in which State Department officials had to decide whether or not to object to drone strikes before the CIA pulled the trigger, the officials said, according to the newspaper.

Law enforcement and intelligence officials said State Department deliberations about the covert CIA drone program should have been conducted over a more secure government computer system designed to handle classified information, the Journal reported.