(Adds comment on possible longer-term tax steps for markets in paragraph 6, background on policy tightening, 7-8)

ISTANBUL, July 1 (Reuters) - A drive by Turkey's government to modernise the country's tax system will seek to boost revenue by tackling tax avoidance and scrapping incentives that are no longer needed rather than raising the overall burden, the finance minister said on Monday.

Mehmet Simsek said, however, that preliminary draft proposals being discussed within the government envisioned a minimum 15% corporate tax on multinational companies, confirming a report last month by state-owned Anadolu Agency.

He did not give further details about the proposal. At present, multinational companies face varying levies depending on numerous factors.

Speaking to local broadcaster BloombergHT, Simsek said the government's plans - which would need to be approved by parliament - also included raising the corporate tax on public-private partnerships (PPPs) to 30% from 25% at present.

Simsek, who has spearheaded a year-long policy-tightening programme to tackle soaring inflation, said in Monday's interview that the tax plan being discussed by government officials was in the early stages and could be subject to changes before being presented to parliament.

He said there were no plans to introduce a transaction tax on the purchase and sale of stocks, but the government could propose taxes on stock market gains sometime in the future.

Earlier this month, an economy official said Turkey had almost finalised work on imposing a transaction tax on the purchase and sale of stocks and crypto assets.

The plans are part of broader efforts to boost government savings, fiscal discipline and price stability after years of turmoil that fuelled soaring inflation.

As part of the tightening programme, the central bank has aggressively hiked interest rates to 50% from 8.5% since June last year. Annual inflation hit 75% in May but was expected to have dipped in June. (Reporting by Ezgi Erkoyun; Writing by Jonathan Spicer; Editing by Daren Butler and Helen Popper)