The weapons, sneaked in for the Shiite Islamic militant group from Syria, include as many as 12 Russian Yakhont supersonic anti-ship cruise missiles, current and former U.S. officials told the Wall Street Journal, citing previously undisclosed intelligence.
Other systems that can target Israeli aircraft, ships and bases are stored in Syrian weapons depots under Hezbollah control, the officials said.
But Hezbollah doesn't appear to have all the components needed to make a complete guided-missile system, the officials said.
"To make it lethal, a system needs to be complete," a senior defense official told the newspaper.
"Hezbollah is pretty damn good," a senior U.S. official added. "And they are patient."
Hezbollah's paramilitary wing is hailed in parts of the Arab and Muslim worlds as a resistance movement against Israel.
But the movement is considered a terrorist organization by the Gulf Cooperation Council of Persian Gulf countries Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates, as well as by the United States, France, Britain, the European Union, the Netherlands, Canada, Australia and Israel.
The wing is also widely considered more powerful than Lebanon's army.
Saudi Arabia pledged $3 billion this week to strengthen Lebanon's armed forces. Analysts said the intention of the record aid package was to shift the balance of power between Lebanon's military and Hezbollah's paramilitary.
Iran -- which has funded Hezbollah since its creation by Muslim clerics after Israel's 1982 invasion of Lebanon -- started pressing for the militant group to upgrade its arsenal in late 2012, U.S. and Israeli officials told the Journal.
Tehran did this out of fear the Syrian regime of President Bashar Assad might be overrun by rebels, the officials said.
Iranian leaders feared if Assad's regime fell, Iran's window for supplying advanced weaponry to Hezbollah could close, Israeli intelligence indicated.
Iran considers Hezbollah's rockets its first line of defense against an Israeli strike, the Journal said.
Smuggling the advanced weapons systems through Syria was also meant to induce Hezbollah to protect Assad and protect supply lines used by both his regime and Hezbollah, U.S. and Israeli officials said.
Hezbollah already has about 100,000 rockets stockpiled, Israel estimates. But those are primarily unguided "dumb" weapons that are far less accurate than self-propelled guided weapon systems.
The new weapons could sharply increase the group's ability to deter Israel in any potential new battle, the officials said.
Israel and Hezbollah had no immediate comment on the report.
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