The comments come a few days before RBS, 83 per cent owned by the UK government, announces second-quarter earnings. Global banking and markets account for more than half of the bank's core profit.
"It wouldn't surprise you to note that the entire fixed-income markets are more challenged in the second quarter than they were in the first and we will be representing that trend," John Hourican, chief executive of RBS's global banking and markets business, told Reuters.
"We have a biased business towards macro, fixed-income. I would have to say for the third quarter the markets continue to be challenging."
Hourican, however, said he cannot guide on the second-quarter earnings.
Deutsche Bank estimates that RBS's core profit will fall 22 per cent quarter-on-quarter, hurt by global banking and markets.
Profits at its core business – retail and investment banking arms and excluding businesses due to be sold off – rose 25 per cent quarter-on-quarter to £2.1bn ($3.4 billion) in the first quarter.
At the group level, RBS posted a loss due to bad debts at its Irish operations.
Sovereign debt fears on both sides of the Atlantic have hurt fixed-income investors as markets worry about defaults and the impact it would have on balance sheets.