Egypt has denounced new U.S. security assurances to Israel as "exceedingly dangerous" to American interests in the Middle East and "Detrimental to the peace process" inaugurated by the Egyptian-Israeli treaty signed here this week.

The sharp protest and a separate warning that the assurances to Israel will damage U.S.-Egyptian cooperation for peace are contained in two letters ent by Egyptian Prime Minister Mustafa Khalil this week to Secretary of State Cyrus R. Vance. The letter were made available to several reporters yesterday.

Signed two hours after the peace treaty on Monday, the U.S-Israeli memorandum of understanding "will cast grave doubts about the real intention of the United States" and is "directed against Egypt," Khalil wrote, Egypt will refuse to recognize the agreement, which requires that both Egypty Israel be consulted before U.S. action t counter a violation is taken.

U.S. officials said they did not expect the protest to affect the peace treaty, which will return the Sinai peninsula to Egypt and establish full diplomatic and commercial relations between two nations that fought four wars in 30 years.

This view was bolstered by the disclosure by diplomatic sources that Sadt has agreed verbally to open land borders with Israel within two months. He agreed to this Sunday night after Begin said Israel would give the norther Sinai coastal town of El Arish back to Egypt in two months.

While noting that the Egyptian opposition would not have legal effect on the bilateral memorandum of understanding, U.S. officials acknowledged that Egyptian negotiators had been upset and surprised by some of the terms of the document, which was signed by Vance and Israeli Foreign Minister Moshe Dyan.

The State Department responded to queries about the two Khalil letters with a press statement noting "an expression of unhappiness" from Egypt, which the State Department contended was based on a misreading of the memorandum. The statement added that the United States was still ready to sign a similar document with Egypt.

The State Department also released the text of the memorandum, which was reported in The Washington Post Tuesday. The nine-point paper specified that in the event of violations that threaten Israel's security, such as naval blockades, the United States "will be prepared to consider, on an urgent basis, such measures as the strengthening of the U.S. presence in the area, the providing of emergency supplies to Israel and the exercise of maritime rights in order to put an end to the violation."

Citing the Post account, Sen. Howard H. Baker Jr. (R-Tenn.) voiced concern about the possibility of U.S. military moves to enforce the treaty. In a speech on the Senate floor, Baker noted. "We are taking steps into the unknown in these momentous days, and we all want to play as constructive a role as possible within the limits of prudent policy."

Although Sadat has frequently expressed the desire for increased American military support for his own government, the military provisions in the memorandum attracted Khalil's sharpest criticism. "It gives the United States the right to impose a military presence in the region for reasons agreed between Israel and the United States, a matter which we can not accept."

Saying that it may strengthen Sadat's Arab critics, Khalil predicted that the agreement will "certainly drive other Arab countries to take a harder position against the peace process, and would give added reasons for them not to participate in that process. "It will also have adverse effects in Egypt for the United States," he wrote.

The letters, written Saturday and Sunday as Vance and Dayan negotiated the final clauses of the memorandum, also express indignation that Egypt's good faith in signing the treaty was being questioned. Khalil complained that the Carter administration, which had expressly agreed to be "a full partner" with Sadat in the peace effort, was not moving to be "an arbiter" of the agreement.

"The government of Egypt will not recognize the legality of the memorandum and considers it null and void," the second letter concludes.

U.S. officials continued to stress their view that the memorandum of understanding does not commit the United States more deeply than does a similar document that then Secretary of State Henry Kissinger gave Israel to underwrite the 1975 Sinai disengagement agreement.

In his final day in Washington, Sadat met separately with Defense Secretary Harold Brown to seek arms and David Rockefeller, head of the Chase Manhattan Bank and one of the nation's most influential financial leaders.

A Pentagon official said that the administration is negotiating a sale of F 4 Phantom fighter bombers to Egypt, which has asked for the more adanced F16s.