ASHKELON, Israel, Nov. 16 (UPI) -- Israel has begun mobilizing 16,000 troops, a move that could signal plans to invade the Hamas-ruled Gaza Strip amid a swelling rocket barrage in which Iranian-made missiles hit the edges of metropolitan Tel Aviv, Israel's largest urban conurbation.
Israeli warplanes kept up a relentless campaign of airstrikes against Palestinian extremists and their rocket depots for the second day, reportedly destroying an estimated 200 medium-range Fajr-5 missiles from Iran and other rockets stored across the densely populated coastal enclave.
The three missiles that hit Tel Aviv were the first to strike the country's commercial capital since Iraqi dictator Saddam Hussein unleashed 36 Scud-B ballistic weapons against it in the 1991 Gulf War.
The attacks on Tel Aviv Thursday and Friday marked a major escalation by the Palestinian militants in Gaza, and underlined the extent to which they have acquired more powerful and longer range weapons since Israel's last invasion in December 2008.
One missile reportedly exploded near Rishon Letzion, about 8 miles south of the center of Tel Aviv.
This has dramatically changed the calculus of conflict since until a few days ago Hamas was only firing short-range Soviet-era 122mm Grad rockets and Qassam rockets produced in makeshift factories in Gaza's teeming refugee camps.
The fighting is likely to intensify. Israeli Home Front Defense Minister Avi Dichter, former director of the internal security service Shin Bet, declared: "We expect the continued firing of hundreds of rockets in the coming days."
Israeli military sources estimate the Palestinians -- the fundamentalist Hamas, which has ruled Gaza since June 2007, and other extremist factions such as Islamic Jihad -- have amassed up to 11,000 missiles and rockets over the last couple of years.
These were largely from Iran, smuggled into Gaza through tunnels from neighboring Egypt's Sinai Peninsula. More recently, weapons have come from Libya following the 2011 civil war there.
However, there's been no sign Hamas has used the Soviet-made SA-7 surface-to-air missiles it's reportedly acquired against the Israeli F-16 strike jets pounding Gaza.
One SAM was fired at an Israeli helicopter several months ago. It missed, but it raised fears the militants were now able to challenge Israel's long-held air supremacy for the first time.
That may yet happen, of course. Even with the Israelis hammering Gaza by land, sea and air, Hamas and the other groups are still able to keep up a steady barrage of rockets.
The Israeli air force claimed to have attacked scores of targets in Gaza since Wednesday, knocking out a large number of projectiles. But the Palestinians have fired an estimated 450 since Wednesday, including 100-plus since Thursday night.
In Israel's 2006 war against the Iranian-backed Hezbollah in Lebanon, the air force knocked out most of the organization's Iranian-supplied long-
range weapons in the first 24 hours.
But Hezbollah, which had an estimated 12,000 missiles of various calibers then, was still able to bombard northern Israel for 34 days, the length of the war, with nearly 4,000 missiles, around 120 a day.
Hamas seems to have a similar capability now.
If the air force cannot shut down the Gaza rocket fire, Prime Minister Binyamin Netanyahu may have to order a major ground offensive that would likely result in heavy Palestinian casualties.
He's empowered to mobilize 30,000 troops for this crisis. That's a third more than the 20,000 that battered Gaza in 2008's Operation Cast Lead.
Military sources say the 16,000 reservists called up so far are mainly from infantry and engineering units.
Given the international furor over Cast Lead, in which 1,400 Palestinians, mainly civilians, were killed along with 13 Israelis, 10 of whom fell to friendly fire, there will likely be substantial international diplomatic opposition to another large-scale incursion.
British analyst Jonathan Freedland noted that Hamas' diplomatic power has grown alongside its arsenal since the 2008 invasion, which was aimed at crushing Hamas to end its rocket fire.
"The Arab revolutions have redrawn the regional map, much of it in Hamas' favor," Freedland observed.
"Once a Pariah, Hamas now sees its own movement, the Muslim Brotherhood, ruling Egypt.
"Where Hosni Mubarak played broker between Israel and Gaza, often seeking to stay Hamas' hand, Cairo's new rulers feel a grassroots pressure to stand as the ally of Hamas."
That might make Israel think twice about invasion.