Michelle Obama has appeared on The Late Show with David Letterman talking about the family's pet dog Bo and campaigning for her husband's re-election ahead of the Democratic convention.

The US First Lady admitts she has not watched any of the Republican convention, but did urge voters to follow both the Republican and Democratic events.

The Republicans are currently gathered in Tampa and the Obamas' turn comes next week when Democrats convene in North Carolina for three days.

Her appearance came on the same night that vice presidential candidate Paul Ryan presented himself to the American public at the Republican event.

The White House said the president is not monitoring the Republican national convention this week.

But he told several thousand mostly college-age voters on Wednesday that they should be watching to help them decide how to vote in November.