The CIA station chief opened the locked box containing the sensitive equipment he used from his home in Tel Aviv to communicate with CIA headquarters in Virginia, only to find that someone had tampered with it. He sent word to his superiors about the break-in.

The incident, described by three former senior US intelligence officials, might have been dismissed as just another cloak-and-dagger incident in the world of international espionage, except that the same thing had happened to the previous station chief in Israel.

It was a not-so-subtle reminder that, even in a country friendly to the United States, the CIA was itself being watched.

In a separate episode, according to another two former US officials, a CIA officer in Israel came home to find the food in the refrigerator had been rearranged. In all the cases, the US government believes Israel's security services were responsible.

Such meddling underscores what is widely known but rarely discussed outside intelligence circles: Despite inarguable ties between the US and its closest ally in the Middle East and despite statements from US politicians trumpeting the friendship, US national security officials consider Israel to be, at times, a frustrating ally and a genuine counterintelligence threat.

In addition to what the former US officials described as intrusions in homes in the past decade, Israel has been implicated in a US criminal espionage cases and disciplinary proceedings against CIA officers and blamed in the presumed death of an important spy in Syria for the CIA during the administration of George W Bush.

The CIA considers Israel its No 1 counterintelligence threat in the agency's Near East Division, the group that oversees spying across the Middle East, according to current and former officials. Counterintelligence is the art of protecting national secrets from spies. This means the CIA believes that US national secrets are safer from other Middle Eastern governments than from Israel.

Israel employs highly sophisticated, professional spy services that rival American agencies in technical capability and recruiting human sources. Unlike Iran or Syria, for example, Israel as a steadfast US ally enjoys access to the highest levels of the US government in military and intelligence circles.

The officials spoke on condition of anonymity because they weren't authorised to talk publicly about the sensitive intelligence and diplomatic issues between the two countries.

The counterintelligence worries continue even as the US relationship with Israel features close cooperation on intelligence programs that reportedly included the Stuxnet computer virus that attacked computers in Iran's main nuclear enrichment facilities. While the alliance is central to the US approach in the Middle East, there is room for intense disagreement, especially in the diplomatic turmoil over Iran's nuclear ambitions.

"It's a complicated relationship," said Joseph Wippl, a former senior CIA officer and head of the agency's office of congressional affairs. "They have their interests. We have our interests. For the US., it's a balancing act."

The CIA declined to comment.

The tension exists on both sides.

The National Security Agency historically has kept tabs on Israel. The US, for instance, does not want to be caught off guard if Israel launches a surprise attack that could plunge the region into war and jeopardise oil supplies, putting American soldiers at risk.

Matthew Aid, the author of The Secret Sentry, about the NSA, said the US started spying on Israel even before the state was created in 1948. Mr Aid said the US had a station on Cyprus dedicated to spying on Israel until 1974. Today, teams of Hebrew linguists are stationed at Fort Meade, Maryland, at the NSA, listening to intercepts of Israeli communications, he said.

Israel is not America's closest ally, at least when it comes to whom Washington trusts with the most sensitive national security information. That distinction belongs to a group of nations known informally as the "Five Eyes". Under that umbrella, the United States, Britain, Australia, Canada and New Zealand agree to share intelligence and not to spy on one another.

Israel is part of by a second-tier relationship known by another informal name, "Friends on Friends". It comes from the phrase "Friends don't spy on friends", and the arrangement dates back decades. But Israel's foreign intelligence service, the Mossad, and its FBI equivalent, the Shin Bet, both considered among the best in the world, have been suspected of recruiting US officials and trying to steal American secrets.

Jonathan Pollard, who worked for the Navy as a civilian intelligence analyst, was convicted of spying for Israel in 1987 when the Friends on Friends agreement was in effect. He was sentenced to life in prison. The Israelis for years have tried to win his release.

The espionage incidents have done little to slow the billions of dollars in money and weapons from the United States to Israel. Since Pollard's arrest, Israel has received more than $60 billion in US aid, mostly in the form of military assistance, according to the Congressional Research Service. The UShas supplied Israel with Patriot missiles, helped pay for an anti-missile defence programme and provided sensitive radar equipment to track Iranian missile threats.

Just on Friday, the US president, Barack Obama, said he was releasing an additional $70 million (Dh257m) in military aid to Israel.