JERUSALEM (Reuters) - Israel has begun construction of an underground barrier along the frontier with the Gaza Strip that is meant to block cross-border tunnels built by Palestinian militants, Israeli defense and political sources said on Thursday.

Since being blindsided during a 2014 war by tunnel raiders from the Hamas Islamist group that controls Gaza, Israel has stepped up work on technologies for spotting the secret passages. Currently Israel has a fence along the border.

Military engineers unearthed and destroyed 32 tunnels during the war, Israeli officials say, and the military has since uncovered two others.

One Israeli political source, speaking on condition of anonymity, said that the government has already budgeted some 600 million shekels ($160 million) to build one section of the underground concrete barrier.

The barrier will eventually be about 65 km in length, the source said.

Israel's Defense Ministry declined to comment on the issue.

($1 = 3.7521 shekels)

(Reporting by Ari Rabinovitch)