By Patrick McGroarty 

PRETORIA--South African officials said on Monday they would use a summit between U.S. President Barack Obama and African leaders next week to lobby for renewal of a trade pact they called an important instrument of U.S. influence on a continent where China and other powers are vying to invest.

"The African continent is the next growth frontier and there is increasing competition to be here among external partners," said South Africa's Trade Minister Rob Davies.

Mr. Davies said the African Growth and Opportunity Act, or Agoa, which African leaders wanted extended for 15 years beyond its expiration date in 2015, is an important leg up for the U.S. in that competition.

"Agoa has generated good will for the United States, and our friendly suggestion is that good will is worth building on," Mr. Davies said.

Mr. Davies will accompany South Africa's President Jacob Zuma to the summit in Washington, D.C. beginning on Aug. 4. Leaders from more than 50 African countries are expected to attend.

Agoa allows African countries with broadly democratic governments to ship goods to the U.S. without paying tariffs. Some countries, like Madagascar and Guinea, have lost their Agoa designation in recent years due to domestic political turmoil. Others, like Lesotho and Kenya, have made great use of the legislation to manufacture clothing and other goods for U.S. retailers.

Mr. Davies said South Africa's trade with the U.S. has also grown dramatically since Congress passed the legislation in 2000. He cited a jump from about $8.4 billion in bilateral trade in 2008 to $12.4 billion in 2013.

But some U.S. officials have suggested that if Agoa is renewed, South Africa be excluded because of its relatively advanced development relative to many of its poorer African peers.

Mr. Davies said excluding South Africa would endanger efforts to form a more contiguous market with its poorer neighbors such as Lesotho and Swaziland. He also said South Africa's robust infrastructure and sophisticated financial sector shouldn't obscure its deep challenges.

In April Nigeria surpassed South Africa as the continent's biggest economy. The central banks says the economy will grow just 1.7% this year, about a third of the rate officials say they need to dent a 25% unemployment rate.

"We have an economy that is actually characterized by high levels of unemployment, high levels of inequality, high levels of poverty," Mr. Davies said, deficits that, he said, increased industrialization driven by trade with the U.S. could help address.

Write to Patrick McGroarty at patrick.mcgroarty@wsj.com