Israel and Fatah Palestinian forces worked together when rival Hamas militants overran the Gaza strip, according to WikiLeaks documents released today.
The disclosure could embarrass Palestinian President Mahmoud Abbas and his Fatah movement, which Hamas has accused of working with the Israelis.
Mr Abbas' standing among Palestinians has already been weakened by his failure to make progress in peacemaking with Israel.
A June 13, 2007, memo from the US Embassy in Tel Aviv, mentions a conversation that took place during the civil war in Gaza that ended with the Hamas takeover.
It says Israeli Security Agency chief Yuval Diskin said he had "established a very good working relationship" with two branches of the Palestinian security service.
Mr Abbas' internal security agency, he said, "shares with ISA almost all the intelligence that it collects".
Palestinians have a complex relationship with Israel, pursuing peace talks on the one hand but considering it an enemy on the other, because of its occupation of the West Bank and its settlements there.
Collaboration with Israeli security is seen by Palestinians as treachery.
The Israeli Security Agency, also known as the Shin Bet, is reviled by Palestinians for its sometimes deadly raids on militant targets and its often harsh treatment of Palestinian suspects.
Although Israeli and Palestinian security forces are known to cooperate, the tight co-ordination described by Mr Diskin could further weaken Abbas.
In the memo, Mr Diskin also said some leaders of Fatah - which he described as "desperate, disorganised and demoralized" - urged Israel to intervene in the infighting in Gaza.
Without identifying the leaders by name, he said they were in an "urgent situation".
"They are approaching a zero-sum situation, and yet they ask us to attack Hamas," Mr Diskin said. "This is a new development. We have never seen this before. They are desperate."
Mr Diskin is also said to oppose a US proposal to supply ammunition and weapons to Fatah, fearful that Hamas might get its hands on them instead.
The message did not suggest that he foresaw Hamas wresting control of Gaza from Fatah. It paraphrased him as saying that while Hamas was dominant in the Gaza Strip, it "is not yet strong enough there to completely destroy Fatah".
In fact, Fatah forces were routed from Gaza in just five days of fighting.
Since the takeover, Abbas' Palestinian Authority has ruled only the West Bank, leaving the Palestinians with two rival governments.
An official with Abbas' government played down the information, saying "information-sharing between us and Israel is limited to field information that serves our security and the interest of our people."