At least 12 people were killed and thousands were wounded in Lebanon on Tuesday when hundreds of pagers exploded simultaneously across the country in a mass attack that appeared to target members of Hezbollah. Another wave of explosions was reported on Wednesday afternoon.

The blasts — which came after Israeli officials said they were ready to step up attacks against the Iranian-backed militia — have heightened concerns that Israel’s long-simmering conflict with the group might escalate into a broader war. Hezbollah and Iran quickly blamed Tuesday’s operation on Israel, an assessment confirmed by U.S. and other officials. Israel has not confirmed or denied responsibility.

With hospitals in Lebanon struggling to cope with the number of wounded, Israel and Lebanon were tensely awaiting a promised retaliation from Hezbollah and its allies.

Here’s what you need to know about the attack and its potential ramifications.

What happened?

Pagers carried by hundreds of Hezbollah fighters began beeping for several seconds at 3:30 p.m. on Tuesday with what appeared to be a message from the group’s leadership, according to two officials familiar with the attack. Then they exploded — in grocery shops, on sidewalks, even on a moving motorcycle.

Lebanese officials said that 12 people had been killed, including a young girl. Hezbollah said that at least eight of its fighters were among the dead. More than 2,700 people were injured in Lebanon — including Iran’s ambassador to the country.

In Syria, at least 14 people were injured, according to the Syrian Observatory for Human Rights, a British-based monitor.