(Adds ships waiting to load Isthmus)
SINGAPORE, Nov 29 (Reuters) - Mexico's national oil company Pemex has declared force majeure on the loading of Isthmus crude, two sources with knowledge of the matter said on Wednesday.
The producer informed buyers late on Tuesday the force majeure would affect cargoes loading in early December, the sources said.
Pemex has offered to replace Isthmus supplies with another Mexican crude grade, Maya, one of the sources said.
The source said force majeure on Mexican crude loadings routinely happens during winter due to poor weather conditions, usually lasting a few days.
Pemex could not be immediately reached for comment as its office is closed during Asian hours.
Four oil tankers are waiting off Dos Bocas port to load Isthmus crude, shipping data on Thomson Reuters Eikon shows.
These include supertanker Maran Ajax and three suezmax tankers Pentathlon, Suez Fuzeyya and Trinity. (Reporting by Florence Tan in SINGAPORE; Additional reporting by Catherine Ngai in NEW YORK; Editing by Tom Hogue)