President Obama called for an "immediate ceasefire" in Gaza in a phone call with Israeli Prime Minister Benjamin Netanyahu on Sunday, and dispatched Secretary of State John Kerry to Cairo to help negotiate one. The White House also released a statement describing the call, effectively making this a public request. The request, which Obama couched in concern about civilian casualties, is a big step-up in the administration's public expression of concern about the Israeli ground offensive in Gaza, launched on Thursday.

This also comes just as the Gaza-based militant group Hamas announces it's captured an Israeli soldier, which if true (and that's a big if) could severely complicate any attempt to broker a peace deal.

Before the fighting between Israel and Hamas had escalated to Israel's ground invasion, the United States had offered to help facilitate a ceasefire agreement. Nothing came of it, and Hamas' military wing later rejected a separate, Egyptian-brokered ceasfire deal. Obama's call for an "immediate" ceasefire, however, indicates greater and more urgent American interest in both sides laying down their arms.

The president 'raised serious concern about the growing number of casualties'

To date, the United States had largely been supportive of Israel's efforts to quell rocket fire out of Hamas, though Obama has publicly, albeit more subtly, warned Israel about civilian casualties. Obama's Sunday call with Netanyahu seems to have gone further. The president, according to the White House statement, "raised serious concern about the growing number of casualties, including increasing Palestinian civilian deaths in Gaza and the loss of Israeli soldiers."

This is the first Israeli ground operation into Gaza during Obama's presidency. The last Israeli incursion ended in a ceasefire on January 18th, 2009 — two days before Obama was sworn in. In late 2012, when Israel launched a series of air strikes against Gaza, the Obama administration was also supportive. But the administration also told Israel that avoiding any ground operation would be "preferable."

It's worth noting here that the call with Netanyahu may well have been a bit frostier than the administration's statement suggests. Obama and Netanyahu have, famously, had a less-than-friendly relationship at points, owing in large part to disagreements over peace process issues like a freeze on Israeli settlements. Obama has pushed for more conciliatory Israeli positions toward the Palestinians; the conservative Israeli prime minister pushed back.

As for the captured soldier, some reports suggest the Israeli military is denying Hamas' claim that anyone has been captured. If Hama's claim turns out to be true, however, it could have potentially huge implications for the present Israel-Gaza conflict. Israel has typically placed a significant premium on recovering captured soldiers, so the soldier's freedom would likely become a major part of both Israeli military operations and any ceasefire negotiations.

Read Vox's full guide to the Israel-Palestine conflict here.