President Nixon announced again last night he would not rule out the possibility of bombing North Vietnam.

In a televised news conference restricted to foreign policy, the President said the use of U. S. airpowers in North Vietnam was contingent on the actions of Hanoi.

The United States will resume air attacks on the North, he said, if infiltration increases as we withdraw our troops, or if North Vietnamese missile sites fire on U. S. troops. But no plan to invade North Vietnam "is presently under consideration in the U. S. government," he said.

Concerning the future of U. S. troops in Vietnam, the President said "as long as there are prisoners of war-there are at present 1600 POW's in North Vietnam-we'll have to keep a residual force there."

The President said he had received news from General Abrams that the South Vietnamese army's activity in Laos has shown they could "hack it" alone, and expressed continued optimism over the success of Vietnamization.

President Thieu released a statement Wednesday that the success of South Vietnamese troops in Laos is based on the presence of the U. S. Air Force.

Nixon said Abrams had further reported to him yesterday that 55 per centof the trucking traffic coming into the south from the Ho Chi Minh trail had been obstructed.

The North Vietnamese announced recently that they were building another major supply route into the south.

In response to several questions by news reporters as to the reported rivalry among his top foreign policy advisors (Secretary of State William P. Rogers and Henry A. Kissinger), the President charged that there is a common practice of trying to create conflict between the President and his chief advisors.

The President labeled the recent criticism by Sen. Stuart Symington (D-Mo.) as a "cheap game." The President cited a host of statistics concerning the exact number of times the Secretary of State had met with the press or representatives of Congress in an attempt to refute the charges.

The President added that he would have another troop withdrawal announcement in April, but predicted that there will be "hard fighting" ahead.