Lawrence Franklin, 58, said in court on Wednesday that he was frustrated with government policy and that he had hoped the two members of American Israel Public Affairs Committee could influence policy with their connections at the National Security Council.
He also admitted giving classified information to a political official at the Israeli embassy, but said the information he received from the official was far more valuable than what he gave.
"I knew in my heart that his government had this information," Franklin said.
"He gave me far more information than I gave him."
Franklin pleaded guilty to two conspiracy counts and a charge of unlawful retention of national defence information.
US District Judge T S Ellis III set sentencing for 20 January.
Franklin, who was one of the Pentagon's policy experts on Iran and the Middle East, was indicted in June on five charges.
The two AIPAC officials who allegedly received the information, Steven Rosen and Keith Weissman, also have been charged with conspiring to obtain and disclose classified US defence information.
AIPAC fired Rosen and Weissman in April. Both AIPAC and Israel deny any wrongdoing.
AIPAC is a pro-Israeli lobby group
with clout in Washington
According to the indictment, Franklin met periodically with Rosen and Weissman between 2002 and 2004 and discussed classified information, including information about potential attacks on US troops in Iraq.
Rosen and Weissman would subsequently share what they learned with reporters and Israeli officials.
On at least one occasion, Franklin spoke directly to an Israeli official.
Rosen, a top lobbyist for Washington-based AIPAC for more than 20 years, and Weissman, the organisation's top Iran expert, allegedly disclosed sensitive information as far back as 1999 on a variety of topics, including al-Qaida, terrorist activities in Central Asia, the bombing of Khobar Towers in Saudi Arabia and US policy in Iran, according to the indictment.
Franklin at one time worked for the Pentagon's No 3 official, policy undersecretary Douglas Feith, on issues involving Iran and the Middle East.