UN humanitarian chief John Holmes said Tuesday he's heading to Gaza. A top priority will be to get all border crossings opened, he said, not only for food and medicine but for desperately needed construction materials which Israel has refused to allow in since Hamas seized power in June 2007. He said "it's absolutely critical" that cement, pipes and other building materials are "unbanned" by Israel and allowed into Gaza to start rebuilding the war-ravaged Palestinian territory. "Otherwise, the reconstruction effort won't get off first base," Holmes said. Holmes, who expects to arrive in Israel on Wednesday, told a news conference he will also be pressing Israeli authorities to allow humanitarian staff from international organizations into Gaza. "In theory, they have permission," he said. "In practice, it's proving very difficult to get into Gaza." Holmes said Monday that hundreds of millions of dollars in humanitarian aid will be needed immediately to help Gaza's 1.4 million people and billions of dollars will be required to rebuild its shattered buildings and infrastructure.