It said a number of regulatory and enforcement authorities were investigating foreign exchange trading, including attempts to manipulate benchmark exchange rates.
Barclays said it was cooperating and was reviewing its foreign exchange trading over a period of several years to August this year.
The group reported total profit before tax for the period down by 26% to £1.38 billion compared to the same period last year - though statutory profits, when some accounting provisions are included, were up sharply.
Barclays said pre-tax profits from its investment bank dropped by 53% to £463 million.
Lower activity was driven by uncertainty about when the US Federal Reserve would start slowing its quantitative easing programme that is pumping billions of dollars into the economy every month.
Profits from UK retail and business banking were 2% down at £351 million.
The latest investigation into rate-fixing comes after Barclays paid £290 million in fines to US and UK regulators in June last year over the manipulation of Libor and Euribor interbank rates, in a scandal that claimed the scalp of chief executive Bob Diamond.
New boss Antony Jenkins, appointed last summer, launched a crusade to overhaul the culture of the bank, with a major restructuring programme called Transform that has cost £741 million so far this year.
But its reputation has taken further blows after it revealed it was facing a £50 million fine over claims it acted recklessly in its multibillion-pound bailouts from Qatar in 2008.
In its latest trading update, the bank said: "Various regulatory and enforcement authorities have indicated they are investigating foreign exchange trading, including possible attempts to manipulate certain benchmark currency rates or engage in other activities that would benefit their trading positions.
"The investigations appear to involve multiple market participants in various countries.
"Barclays Bank has received enquiries from certain of these authorities related to their particular investigations, is reviewing its foreign exchange trading covering a several year period through August 2013 and is cooperating with the relevant authorities in their investigations.
"It is not possible at this stage for Barclays to predict the impact of these investigations on it."
Elsewhere in the report, Mr Jenkins said the group was "well-positioned to take advantage of improvements in the global macro environment".
He said performance was resilient, but added: "I am not complacent, and my executive team know we must push harder in the final quarter into 2014."
Last month, the bank received strong support for a £5.8 billion rights issue offered as part of a plan to meet the City regulator's demand that it plug a £12.8 billion hole in its finances.
Mr Jenkins said in the latest update that plans to meet the demands by June 2014 were on track.
In its outlook statement, the bank said: "We continue to remain cautious about the environment in which we operate and our focus remains on costs, capital leverage and returns to drive sustainable performance improvements."
Shares were up nearly 3%. Analysts at Credit Suisse said the results were ahead of consensus forecasts.