A unilateral ceasefire called for by Kiev is under pressure after pro-Russian separatists attacked an Ukrainian military base and posts on the border with Russia, government forces said.
The fresh attack came just hours after the start of a ceasefire at 10 p.m. on Friday by Ukrainian forces, ordered by President Petro Poroshenko as part of his plan to end the rebel insurgency in the east of the country.
More top news
Some 66 people have been killed after the plane they were travelling on crashed in southern Iran.
The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".
Police say that anyone who sees Whitehouse should not approach him, but instead call them on 999 and quote incident 23 of 18 February.