He said the fact that Israel was holding back from an invasion of Gaza, and that rocket attacks on Israel were lessening, were "positives" in an otherwise desperate situation.
Mr Hague was speaking after talks in Brussels in which EU foreign ministers voiced full support for continuing international peace efforts.
About 95 Palestinians and three Israelis are reported to have been killed by Israeli air strikes on Gaza which began last week in response to a wave of rocket attacks on Israeli towns and cities from the Palestinian enclave.
Leaving Brussels, Mr Hague paid tribute to Egyptian efforts to negotiate "a ceasefire that can work".
He went on: "I am pleased that Israel has held back from a ground invasion while such negotiations go on, and that the rate of rocket attacks on Israel has fallen, for whatever reason, over the last 24 hours. These are positive developments, but of course it remains a desperately serious and difficult situation".
Mr Hague said everyone across Europe remained gravely concerned about Gaza, and EU governments, including the UK, fully supported continuing peace efforts.
A declaration from the foreign ministers emphasised Israel's "right to protect its population" but said any response to rocket attacks from Gaza had to be proportionate.
Foreign Office minister Alistair Burt has arrived in Israel for a three-day visit, during which he will also go to the Gaza Strip. He will meet senior Israeli ministers and Palestinian president Mahmoud Abbas and prime minister Salam Fayyad.