A Member of United Nations Interim Force in Lebanon (UNIFIL) stands guard next to the reported location of the launching of the four rockets at Israel, on the outskirts of the southern Lebanese port city of Tyre on August 22, 2013. (MAHMOUD ZAYYAT/AFP/Getty Images)

U.S. officials believe Hezbollah is smuggling advanced missile parts into Lebanon from Syria one by one, with the goal of upgrading their missile threat to Israel, the Wall Street Journal reports. The piecemeal smuggling is an attempt to evade Israeli air efforts to deter precisely such weapons movement.

Some components of a powerful antiship missile system have already been moved to Lebanon, according to previously undisclosed intelligence, while other systems that could target Israeli aircraft, ships and bases are being stored in expanded weapons depots under Hezbollah control in Syria, say current and former U.S. officials.

According to the Journal’s report, there are two main reasons for the large-scale stockpiling effort, which involve both Iran and Syria.

Iran wants to upgrade Hezbollah’s arsenal to deter future Israeli strikes—either on Lebanon or on Iran’s nuclear program, U.S. and Israeli officials say. In addition, these officials said they believe the transfers were meant to induce Hezbollah to commit to protect Syrian President Bashar al-Assad as well as supply lines used by both his regime and Hezbollah.

Meanwhile, tensions in the country’s Hezbollah stronghold of Beirut are heightened after the explosion of a car bomb on Monday—the second in less than a week—which killed at least five people and injured more than 70.