JERUSALEM (Reuters) -- Iran and its Lebanese ally Hezbollah have built up a 50,000-strong army of Syrian militiamen to help the army keep President Bashar Assad in power, Israel's military intelligence chief said on Thursday.

Major General Aviv Kochavi said the two Israeli foes, in addition to providing Assad's forces weapons and intelligence to defeat rebel fighters, are also hedging their bets by trying to establish a foothold in Syria for after his regime falls.

Around 70,000 people have been killed in Syria's 23-month-old conflict, which grew into an armed revolt after street protests against four decades of autocratic rule by Assad and his late father were met by live ammunition.

The war has worsened sectarian tensions, and Kochavi, speaking at the Herzliya conference on strategy and diplomacy, described a Syria crumbling into ruins.

Since last summer, a "people's army", financed by Iran and trained by Hezbollah has been established in Syria, he said.

"This army consists of about 50,000 people, deployed in a hierarchy around Syria, operating beside the army units, which are collapsing or not collapsing in different places. And the plan is to increase this army to 100,000 people," Kochavi said.

Kochavi said Assad is facing an opposition force of about 100,000 people. Also fighting the government is a separately financed group of about 10,000 radical jihadists, many armed with "relatively advanced weapons", he said.

Iranian-backed Hezbollah, one of Lebanon's strongest factions, is a major ally of Assad, whose minority Alawite sect is an offshoot of Shi'ite Islam.

Iran and Hezbollah realize that Assad will soon fall, Kochi said, and are making preparations for the day after.

"The people's army that they are building is, among other things, a foot in the door to Syria. In the first stage, to protect their Shi'ite interests, in the second stage their assets, and the third stage, from within this army will emerge a leadership to vie to lead Syria after Assad's fall," he said.