One day after President Mohammed Zia ul-Haq vowed his army would defend Pakistan's borders against Soviet invasion from occupied Afghanistan, the military governor of this frontier province today said the Soviets could strike here "with impunity."

Lt. Gen. Fazal e-Haq said that Pakistan has allowed its western defenses to deteriorate to such a degree that the Soviets could attack and withdraw into Afghanistan before Pakistan's Air Force, based 30 miles east of here in Peshawar, could mobilize.

The Carter administration, in response to the threat posed by the Soviet presence in Afghanistan, has portrayed a militarily strong Pakistan as the key to stabilizing Central Asia against further Soviet expansion. Early this week, administration officials said the United States has offered Pakistan a new $400 million aid program, to be split over a two-year period between military and economic assistance.

But high-level Pakistani officials today made clear they feel $400 million "is not meaningful at all." One official said it would cost about $1 billion just to replace Pakistan's outmoded and dated F86 jet fighters.

Fazal, who is both the military commander of this region and the governor under the Army-run martial law government, was unusually frank in his pessimistic assessment of Pakistan's western defenses with Afghanistan.

But his candor, in conversations with correspondents accompanying British Foreign Secretary Lord Carrington on a tour of the frontier area, may have stemmed from Pakistan's desire to increase the level of U.S. military assistance.

In interviews here and later at the governor's mansion in Peshawar, Fazal said that Pakistan had allowed its western defenses to run down because it had concentrated its armed forces on its eastern border with India.

He said the rugged hills and valleys of this border make Pakistan's radar virtually useless for detecting either low-flying and high-flying planes.

They can attack five miles or more inside Pakistan and get away with it before anyone knows what happened," Fazal said.

His stroops are ill-equipped to face a major army, Fazal said. They have "pretty ancient" equipment of Korean War vintage, most of it given by the United States in the 1950s.

Fazal commands two infantry divisions -- about 20,000 troops as well as another 20,000 frontier forces that include the famous Khyber rifles formed 102 years ago by the British to defend the frontier from attack by Afghanistan.

The United States has estimated Soviet strength in Afghanistan at 85,000 troops.

According to one Pakistani official, the U.S. military agrees with Pakistan on the amount of aid it needs, but the State Department has felt Pakistan may be more vulnerable to subversion or political instability than military attack.

This official said U.S. diplomats have repeated that view since April 1978, when a Marxist government took over in Afghanistan. Now, he said, Pakistani predictions of the Soviet troops moving to the Khyber Pass have come true.

Fazal acknowledged that Pakistan has allowed the British-built defenses in this area to go to ruin. He said the bridges are unable to bear the weight of tanks, and the hills are so steep come through its soft underbelly south of here, Baluchistan.

In Baluchistan, the ground is level, making it easier for tanks to operate. It is mostly sparsely populated desert, and the Pakistani general said lack of water may be the biggest problem for an invader. Fazal said that Baluchistan was even more lightly defended than this Northwest Frontier Province.

Fazal said that Afghan planes have violated Pakistani territory many times in the past. Since the Soviets moved into Afghanistan in force on Dec. 26, he said, there have been three photo reconnaissance missions study-that the railroad to the Afghan border can only take five cars at a time.

The tiger's-tooth tank traps set by the British during World War II still are strewn by the side of the highways, but the underground defense bunkers need extensive work before they can be used, he said.

Fazal said the defenses could be put in shape in about six months if Pakistan is given the finances and the bridges could be strengthened in about the same time.

Because of the natural barrier provided by the mountains, he predicted that any attack on Pakistan would [TEXT OMITTED FROM SOURCE] ing routes through these passes and the highway and bridge system here.

The general denied that Pakistan is arming or training rebel forces fighting in Afghanistan, though he acknowledged there is no way for the government to keep the rebels from moving between the two countries over about 3,500 clandestines routes threading across the tribal belt where the central government has little control.

Fazal predicted the Soviets would spend the next two to three months consolidating their position in Afghanistan. But by April, when the snows have melted, they will unleash a major attack on rebel mountain strongholds.

He said that campaign will cause another massive exodus of refugees to Pakistan, and by spring the number will jump from the present 400,000 to close to 1 million.

Lord Carrington, meanwhile, pledged British support to the Pakistanis as he left here for India to try to persuade that country that arms to Pakistan do not threaten it.