SIRKIN SPECIAL FORCES BASE, Israel, March 8 (Reuters) - Fearing a surge in tunnel and bunker construction by Lebanon's Hezbollah and Palestinian militants, Israel is training its troops to hunt below ground with robot probes and sniffer dogs.

Such preparations reflect concerns that should Israel or the United States carry out attacks on Iran's disputed nuclear programme, Tehran may retaliate through its allies on the Jewish state's borders.

"You can see a growing threat," said a major from the Israeli engineering corps' Yahalom ("Diamond") commando unit.

"We're gathering the information, studying it and building training facilities to train our soldiers," the officer, whose name was withheld under secrecy regulations, told Reuters Television during an exercise put on for the foreign media at Sirkin special forces base in central Israel.

The Yahalom men were armed with pistols and snub-barrelled assault rifles to ease movement through narrow passages. Oxygen masks are an option, should ventilation prove problematic.

After the soldiers blew up the door of a mock tunnel, a camera-carrying robot was pulled out of a backpack and tossed inside. A "battle" -- with blanks fired -- ensued against comrades playing Hezbollah fighters.

The Israelis use dogs to sniff out booby-traps in the tunnels and, if needed, to tackle the defenders.

Outgunned and outnumbered, Hezbollah dug tunnels to hold off Israeli ground forces during the 2006 Lebanon war. Israel suspects the Iranian-backed militia has since built underground networks to carry out ambushes in any future conflict.

"Israeli troops were completely taken by surprise at the extent and sophistication of these underground systems," said Nicholas Blanford, a Beirut-based Jane's analyst and author of "Warriors of God: Inside Hezbollah's Thirty-Year Struggle Against Israel".

Blanford said the tunnels enabled Hezbollah to launch rockets into Israel in 2006. "Seeing as they had some bunkers extremely close to the border, it would not surprise me at all if they had burrowed under the border for use at a later date," he added.

RESISTANCE

Palestinian militants in the Hamas-ruled Gaza Strip used a cross-border tunnel to capture Gilad Shalit, a soldier for whose return, after more than five years in captivity, Israel last year freed more than 1,000 jailed Palestinians.

"We are ready to confront them above ground and underground, and such propaganda and drills do not frighten us," said Abu Attaya, spokesman for the Popular Resistance Committees faction, of the Israeli media exercise.

He called Gaza's tunnels "tools of the resistance".

Palestinians also use the underground networks to smuggle building materials, energy supplies and weapons into Gaza from Egypt, which cooperates with an Israeli blockade on the enclave.

Demolition is among Yahalom's specialties, and its major said destroying tunnels and bunkers with explosives was a "good solution".

"But there are cases where the real mission of the force is getting in the tunnel, exploring it, maybe taking out high-value intelligence or releasing a kidnapped soldier," he said.

"We prefer not to get in, but if we have to, we know how to do it."