Several Palestinian militants who either fled or were expelled by Israel have returned to Gaza via the recently re-opened Rafah border crossing.
Fadel Zahhar (L) was expelled by Israel in 1991
The entry of as many as 15 members of Hamas, including one of its founders, has angered Israel.
Israeli Defence Minister Shaul Mofaz has warned his government will close two crossings it controls if militants continue to enter via Rafah.
The crossings into Israel are vital for trade between Gaza and the West Bank.
The Hamas members allowed to return via Rafah include one of the group's founders, Ahmed al-Malah, and Fadel Zahhar, a brother of Hamas leader in Gaza, Mahmoud Zahhar.
Fadel Zahhar returned to Gaza after 14 years in exile, having been expelled to Lebanon by Israel in 1991.
Palestinian officials insist that anyone with a Palestinian identity card can enter Gaza from Rafah.
Israel closed the crossing in September shortly after withdrawing from Gaza, citing concerns that it would be used to smuggle weapons and militants from Egypt into the Palestinian Territories
GAZA CROSSINGS DEAL
Palestinians in charge of crossing, but authority limited
EU monitors have power to detain individuals and vehicles
Israel can watch crossing on television screens
Israel can object to particular travellers, but cannot veto passage
Imports from Egypt must go via Israeli checkpoint at Kerem Shalom
Palestinian bus convoys between Gaza and West Bank from December, and lorry convoys from January
Gaza sea port to be constructed
On Thursday, Mr Mofaz threatened to expel Gaza from a vital customs agreement that allows it to trade with the Palestinians in the West Bank.
"If it doesn't improve and the Palestinians don't co-operate we will close the Erez and Karni crossings," he said.
"They will become international crossings in all senses, and I really hope that the Palestinians understand the significance of this step."
Israel has also complained that the Palestinian Authority is not letting it monitor the Rafah crossing as agreed.
Israel allowed the border to re-open only if they could watch real-time footage on television screens from a base a few kilometres away.
The US Secretary of State, Condoleezza Rice, has responded to the developments by sending a senior envoy to monitor the agreement she brokered last month.
State Department Counsellor Philip Zelikow will examine the operation of the crossing.
He will also discuss upgrading the Erez and Karni crossings and running bus and lorry convoys between Gaza and the West Bank.