Egyptian Foreign Minister Ahmed Aboul Gheit set the condition in a response to Tibor Toth, the new executive secretary of commission which oversees the test ban treaty, it said on Saturday.
"The minister said that Egyptian ratification of the treaty was linked to the extent of developments that may occur in regional and international circumstances, including the possibility that Israel may join the NPT," the agency said.
Egypt is one of 44 states that are deemed capable of producing nuclear weapons and that need to ratify the 1996 test ban treaty before it comes into force. Eleven of the 44 have not ratified, including Egypt, Israel and Iran in the Middle East.
The treaty bans all nuclear weapons tests, while the non-proliferation treaty aims to prevent countries developing nuclear weapons.
The Egyptian agency said Toth wrote to Egypt to say he hoped Cairo would sign the treaty in time for a conference in New York in September.
Aboul Gheit replied that a nuclear danger continued to threaten the region and every state in the region except Israel had signed and observed the non-proliferation treaty.
Israel has never admitted it has a nuclear weapons programme but is widely thought to have about 200 nuclear warheads.