Pakistan is seeking "several billion dollars" in U.S. military aid to build up its defenses along its western border with Afghanistan, a senior Pakistani official said today.

In the latest high-level statement on the issue, the official repeated Pakistan's demand for a new security treaty with the United States to face the perceived threat from last month's Soviet invasion of Afghanistan.

The Carter administration has proposed a $400 million military aid package over two years for Pakistan. But as a result of the apparent inability of the two sides to agree on the size of the package, the administration has postponed sending its request to Congress for approval.

As the debate over U.S. aid continued, reports of pro-Soviet demonstrations in the southwestern province of Baluchistan were reaching Islamabad.

Meanwhile, President Mohammed Zia ul-Haq's foreign adviser, Agha Shahi, lashed out today at newly elected Indian Prime Minister Indira Gandhi for criticizing U.S. plans to aid Pakistan while refusing to denounce the Soviet intervention in Afghanistan.

Agha Shahi, Zia's foreign affairs adviser and the de facto foreign minister, said, "The development of closer relations with the Soviet Union cannot be resumed as long as the Soviet military presence in Afghanistan continues."

He criticized Gandhi only a week before the scheduled visit to Pakistan of her foreign minister for talks on the Soviet intervention.

"Mrs. Gandhi's statements in regard to the Soviet intervention in Afghanistan and the prospects of Pakistan receiving military and economic assistance have been negative and far from reassuring," Agha Shahi's statement said. Ganhi has refused to denounce the Soviet intervention, but has condemned U.S. proposals to aid Pakistan militarily.

India abstained in the U.N. General Assembly vote eight days ago that demanded withdrawal of foreign troops from Afghanistan.

Two days later, Gandhi told reporters, "I don't think that any country is justified in entering another country.I don't think we supported the [Soviet] action."

Agha Shahi's statement avoided any scornful characterizations of the $400 million U.S. aid proposal, which Zia has called "peanuts." In considerably toned-down language, the statement noted that "the assistance must be commensurate with the size of the threat" that it said was "developing on [Pakistan's] western frontier."

Agha Shahi later told a group of reporters that China has agreed to continue military aid to Pakistan, but that "Chinese ability to supply arms to Pakistan is limited."

The presidential adviser denied, however, that there was any move afoot for a "Washington-Islamabad-Peking axis."

In other remarks, the senior Pakistani official bristled at U.S. suggestions that his government was trying to milk Washington for all the aid it can get while concern about the Soviet intervention is running high.

"We are asking very serious questions," he said. "We are not playing hard to get."

After insisting Pakistan would need "several billion dollars" to build up its western defenses, the official added, "We don't say the United States should here and now give the whole amount. But as a beginning what they have offered is far to little."

Zia has said his armed forces need modern planes as a defensive weapon, as well as ground-to-air missles, anti-tank weapons and better communications equipment.

The senior official reaffirmed Zia's call five days ago for the United States to covert its 1959 security agreement with this country into a long-term treaty that would guarantee Pakistan's freedom and integrity.

He said Pakistan wants a "long-term treaty with the United States" giving firm defense commitments. The official said Pakistan "is now in the process of asking all the vital questions" to see if Washington is prepared to sign one. So far the Carter administration has ruled out any new treaty, asserting that the 1959 agreement with Pakistan is adequate.

The senior Pakistani official charged that Washington has not lived up to its obligations under that agreement in the past and "reverses the right to interpret the treaty any way it wants."

"We are sick of depending on the political whims of the United States and U.S. public opinion, which from time to time puts Pakistan in the doghouse," the official said.

Western diplomatic sources, meanwhile, reported that Pakistan has sent troops to its southwestern province of Baluchistan to deal with pro-Soviet demonstrations in the provincial capital, Quetta, and other Baluchi towns. The sources said the demonstrations had taken place in the last couple of weeks, but declined to elaborate.

It was the first indication that the Soviet invasion of Afghanistan may be stirring up leftist Baluchi separtists seeking to carve a "Greater Baluchistan" out of that tribal region straddling the Iranian-Pakistani border along the Arabian Sea and the Gulf of Oman. Baluchi tribesmen in the Pakistani portion fought a four-year guerrilla war against the central government here until a truce was reached in November 1977.

Some Marxist Baluchi leaders now are reportedly, based in Afghanistan south of Kandahar and Pakistani leaders fear that the Soviet intervention in that country may entail Soviet support for renewed separatist activity by leftist Baluchis.