May 20 (Reuters) - Brazilian pulp and paper company Suzano has been in talks with its advisers about sweetening its $15 billion acquisition offer for International Paper people familiar with the matter said on Monday.

Reuters reported earlier this month that Suzano had approached International Paper to verbally express interest in an all-cash acquisition that was worth roughly $42 per share. Such a deal is conditional on International Paper abandoning its agreement to acquire British packaging firm DS Smith for $7.2 billion

International Paper rejected Suzano's initial approach, and Suzano is discussing raising its offer by a few dollars per share, one of the sources said.

The sources cautioned there is no certainty that a new bid from Suzano would succeed, and asked not to be identified because the matter is confidential.

Suzano and International Paper did not immediately respond to requests for comment. (Reporting by Anirban Sen and David Fench in New York Editing by Greg Roumeliotis)