Israeli police questioned a former minister and a political advisor Monday as a probe widened over suspected corruption in the purchase of submarines for Israel's navy from Germany's ThyssenKrupp.

The remand of Eliezer Sandberg, who was science minister in 2003 and served as infrastructure minister in 2004, was extended till Wednesday after questioning, police said.

Rami Tayeb, an advisor to Energy Minister Yuval Steinitz, was also questioned and kept in remand until Wednesday, police said.

Police likewise questioned a third suspect, identified only as a former senior member of Israel's National Security Council.

The questioning came a day after six people were detained in connection with the case, including a former chief of the office of Prime Minister Benjamin Netanyahu.

In July, Germany delayed signing a deal with Israel for the sale of three submarines, an Israeli official said, as the corruption probe gained momentum.

That followed the arrest of several people on suspicion of offences including bribery and money laundering around the deal to buy the Dolphin submarines from the German industrial giant.

David Shimron, a relative of Netanyahu and his family lawyer who also represented ThyssenKrupp in Israel, was among those questioned and then released in July.

Israeli officials said Germany had not backed out of the deal but was waiting to see the outcome of the investigation, according to Yediot Aharonot newspaper.

In February, Israel's justice ministry said it had launched an investigation into the affair, stressing that Netanyahu himself was not a suspect.

Media reports said Sunday's arrests follow revelations by ThyssenKrupp's Israel representative Michael Ganor.

He was held in custody and in a plea bargain agreed to testify against those he claimed were his accomplices.

The submarines are primarily intended for spying missions off Iran or to attack the Islamic republic in case of nuclear war, according to foreign military experts.

Maariv newspaper says the Israeli navy already has five state-of-the-art German submarines.