Hamas leader Khaled Mashaal is visiting the Gaza strip for the first-ever time, on his first trip to the occupied Palestinian Territories for 45 years.
Mashaal is due to stay for three days, to attend Hamas' 25th anniversary rally on Saturday.
His visit comes two weeks after the bloodiest round of fighting between Israel and Gaza in four years.
Hamas has portrayed itself as the victor because Israel agreed to a truce after eight days, and did not send in ground troops as it threatened.
This weekend's anniversary rally will be celebrated as a celebration, with thousands expected to attend.
A local Hamas leader said:
Mashaal is coming at a time when we are celebrating victory in the war. A Hamas leader should come and celebrate with his people.
More top news
The PM will sound out the possibility of new free trade agreements with Gulf states for once the UK has left the EU.
In the unexpected announcement on Monday, Mr Key said it was the "right time" to leave politics.
As the Government's Brexit appeal comes to the Supreme Court, here's a look at what you need to know.